The SALT II agreement announced yesterday represents an attempt by the two nuclear superpowers to put limited controls on the strategic arms race without significantly disturbing either country's nuclear forces.

The Soviet Union and the United States have not agreed on ways to halt or reverse that arms race, so both sides will continue to spend heavily on strategic weapons programs - including new missile-carrying submarines, missiles, warheads and bombs - even if the strategic arms limitation treaty is ratified by both countries.

But the new agreement will cut off some avenues of competition, and will, in the publicly expressed opinions of both governments, provide a basis for continued negotiations. Both have expressed hope that these could produce more substantial reductions.

If ratified, the new agreements would leave nearly intact the existing nuclear forces on both sides. Each country will retain an aggregate total of about 2,000 land-based intercontinental missiles, submarine-based intercontinental missiles and long-range bombers all armed with thermonuclear weapons. To meet the numerical limits in the new agreement, the Soviets would have to dismantle 150 outdated missiles by 1982.

At the outset the United States would retain its nearly 2-t0-1 advantage in deliverable nuclear warheads (nearly 10,000 to nearly 5,000), but the Soviets would be able to steadily close that gap during the life of the treaty, which would remain in force until 1985.

The first SALT agreements, signed in Moscow in May 1972, included one fundamental breakthrough toward eventual arms control. The two superpowers agreed to forgo defensive, anti-missile missile systems that could theoretically destroy the other side's rockets in mid-flight, before they could land and detonate their thermonuclear weapon.

In effect, the Soviets and Americans agreed that each country would be vulnerable to the offensive rockets of the other. Theoretically, at least, this shared vulnerability discourages both sides from considering the use of nuclear weapons.

If SALT II contains a breakthrough (which is a matter of dispute), it is the limits on introduction of new weapons systems and the ceiling on the number of thermonuclear warheads that each country can aim at the other.

Both countries are limited to introducing a single "new type" of land-based missile during the life of the proposed treaty. Proponents argue that this qualitative control could measurably slow the arms race and significantly restrict the Soviets' ambitious development of ICBMs.

The ceiling on thermonuclear warheads would, for the first time, fix an absolute maximum on the number of bombs each country could put on its rocket forces. That total will be extraordinarily high - theoretically it could reach more than 17,000 on each side. In the 1960s then-Defense Secretary Robert S. McNamara estimated that several hundred well-aimed thermonunclear weapons could effectively destroy both United States and the Soviet Union as functioning industrial societies.

Nonetheless, it is a fixed limit, the first in the nuclear era, and proponents of SALT argue that without this quantitative control, even at a high level, the dangers of future escalation of the arms race could be enormous.

Some critics of SALT II have argued that both the qualitative and the quantitative limits are unfair to the United States, since they permit the Soviets to maintain a bigger and more varied force of land-base missiles (which they already possess) and to arm their rockets with heavier, potentially more effective nuclear bombs (because their rockets are bigger, and can carry heavier warheads).

Here is a summary of main provisions of the through-1985 SALT II:

Numerical Limits. Each side would be limited initially to 2,400 launchers of strategic weapons - land-based missiles, submarine-based missiles, heavy intercontinental bombers and bombers carrying cruise missiles. This totals would fall to 2,250 by 1982.

The treaty does not limit missiles as such, but rather missile launchers - the silos, tubes or vehicles from which a missile can be launched. The two sides agreed on this because spy-in-the-sky satellites can spot launchers, while missiles could be stored in warehouses or elsewhere out of satellite camera sight.)

Twelve hundred of these launchers can be for missiles carrying multiple, independently targeted reentry vehicles (MIRVs). Existing MIRV'd missiles can carry from three to 14 separate thermonuclear warheads, each of which can be directed with great accuracy at a separate target.

Of those 1,200 launchers, 820 can be for land-based missiles with MIRVs. Land-based missiles are potentially the most deadly because they are fired from fixed points, and hence can achieve the greatest accuracy. Missiles fired from submarines must rely on navigation systems that are inherently less acurate, though the United States now believes it can produce a submarine-launched missile virtually as accurate as a land-based one.

Both sides are also permitted an additional 120 strategic weapons, but only in the form of heavy bombers that carry an average of 28 cruise missiles each. In practice, only the United States will use this allowance. The American military is developing cruise missiles with guidance systems that "read" the terrain below with a computer brain as they fly, enabling them to strike targets with great accuracy. The idea is to launch these missiles from bombers that remain outside Soviet airspace.

The United States plans to deploy about 135 of these new weapons, using the 120 allocation plus about 15 slots from the overall limit on MIRV'd missiles. The Soviet Union is far behind in this technology.

The remainder of the strategic weapons permitted each side will be intercontinental bombers and missiles carrying non-MIRV warheads.

In connection with the numerical limits in the agreement, both countries have accepted certain "counting rules" to facilitate verification. The most important of these provides that, once a missile has been tested with MIRV warheads, every missile of that type will be considered MIRV'd for the purposes of the SALT II agreements.

Qualitative controls. SALT II includes an elaborate definition of a "new type" of missile substantially different in any basic characteristic from a missile already in use is "new." Each side can deploy only one "new" land-based missile during the life of the treaty.

This will allow the United States to build and deploy a new MX land-based missile. How the Soviets will use this option remains to be seen.

In recent years much of the improvement of both sides' forces has come from the addition of new warheads. SALT II will not freeze this process, but it does set an upper limit of 10 warheads on each MIRV'd land-based missile and 14 on each MIRV'd submarine-based missile.

Existing missiles that remain deployed during the life of the treaty cannot have any more warheads than they do now; none can be added.

Missile size is also limited. The Soviets are permitted to keep their existing force of 308 extra-large "heavy missiles," but any new missile either side builds must be about the size of a Soviet SS 19 in terms of the payload it can carry. The United States reckons this payload ("throw weight" in the jargon of SALT) to be 3,500 kilograms.

Confidence Builders. Several provisions are intended to enhance each country's confidence in its ability to know what the other side is doing. One involves the first-ever exchange of relevant information by both sides.

SALT I was negotiated on the basis of numbers provided by the United States; the Soviets refused to say how many weapons of various kinds they actually had. This time each country has agreed to provide the other what is being called a "data base" - facts and figures on its nuclear arsenal.

Also SALT II provides that each plans to test a long-range land-based missile, unless it is a single test along established test ranges within one country's territory (an exception that applies in practice only to the Soviets).

Verification. SALT II retains SALT I's ban on iterference with "national means of detection" - satellites and other listening and watching devices by which each country can verify that the other is respecting the treaty.

One possible ambiguity here involves "telemetry," the radio signals a rocket and its warhead send back to Earth during a test flight. An understanding accompanying the new agreements says that both countries realize it would violate the treaty to encode telemtry that is relevant to verifcation of the agreement. A dispute seems possible over the term relevant.

The debate that has already erupted publicly over American verificatiion ability does not relate specifically to the terms of SALT II, but rather to the adequacy of spy satellites and other eavesdropping devices to insure that the Soviets are not cheating.

Exceptions. The treaty does not cover medium-range nuclear weapons based in Europe, including the nuclear forces of Britian and France. A medium-to-long-range Soviet bomber known as Backfire is put in this category for purposes of SALT II, so it is not counted. The Soviets will provide assurances not to increase the rate of production of this bomber, now about 30 a year. The United States retains the right to build a bomber similar to Backfire.

Protocol. The new agreement includes a protocol that will remain in force until Dec. 31, 1981.

The protoco bans testing or deployment of mobile long-range missiles. The United States is condsidering a possible mobile basing system for its new MX missile, but could not have it ready to test before December 1981 in any case. The Soviets appear to be farther along in the development of mobile missiles.

The protocol also bans the deployment of cruise missiles launched from ships or ground taht have a rang e of more than 360 miles. A ground-launched cruise missile with a longer range than that could be an appealing weapon for use in central Europe, but U.S. sources say it too could not be ready before the end of 1981, when the protocol would expire. The protocol permits development work to continue.

Finally, the protocol bans testing or deployment of long-range ballistic missiles fired from airplanes.

SALT II also includes a statement of principles for negotating SALT III. It says those negotiations should begin as soon as this treaty is ratified, and should lead to "significant reductions" in the levels of permitted strategic weapons; no numbers are specified. SALT III should also include additional limitations on qualitative improvements of weapons, further limits on strategic defenses, and "cooperation, presumably involving some sort of on-site inspection, the statement says. CAPTION: Chart, U.S. intelligence compares present and future strategic arsensals if SALT II (expiring in 1985) is approved. If rejected, Soviet weaponry might expand as much as 30 percent, some official projections say. By Richard Furno - The Washington Post