Following is an explanation of President Reagan's decision on compliance with the unratified SALT II treaty.

Q: What did President Reagan say about the SALT II treaty and why has it caused so much controversy?

A: Reagan announced May 27 that the United States will no longer base decisions about its strategic military forces on "standards" contained in the structure of strategic arms agreements with the Soviet Union, and especially on the 1979 SALT II treaty, which was signed by President Carter but never ratified by the Senate. This was interpreted by many people as the junking of the treaty.

Q: Wasn't Reagan always opposed to the SALT II treaty?

A: Yes. He condemned the treaty as "fatally flawed" while a presidential candidate and has never said a good word about it. However, until last week, Reagan said he would "not undercut" the treaty by exceeding its limits on strategic nuclear weapons or otherwise violating it as long as the Soviet Union does the same.

Q: Does last week's statement mean the United States is now in violation of SALT II limits?

A: No. Reagan said he has ordered the dismantling of two older Poseidon nuclear missile submarines as a new U.S. Trident missile submarine goes to sea. This will keep the United States under SALT II numerical limits for several months. Reagan said he is doing this for "military and economic" reasons -- not because he wants to continue complying with SALT II.

Q: Is the United States planning to violate SALT II?

A: Reagan said that near the end of this year he "intends" to order the deployment of the 131st B52 bomber armed with air-launched cruise missiles. This would exceed the limits specified under the treaty unless some other U.S. strategic weapons are dismantled to compensate, and Reagan said he does not intend to do this.

Q: Is there still a possibility that the United States might not violate the treaty?

A: Reagan said he hopes that the Soviet Union will now "take the constructive steps to alter the current situation," adding that if it does so, he would "certainly take this into account."

But Reagan did not say precisely what he wants the Soviets to do or what he meant by saying he would take it "into account." No formal statement on these points has been made. Administration officials have suggested that, for Reagan to reconsider his stand, the Soviets must, at a minimum, reverse actions that the United States has charged violate SALT II.

Q: What are the Soviet violations Reagan is talking about, and what is their significance to his decision?

A: Reagan cited two violations by the Soviets and said their noncompliance with the treaty was the basic reason for the change in U.S. policy.

The two violations he cited:

*Soviet deployment of the SS25, a land-based intercontinental ballistic missile. The 1979 treaty permits each side to deploy only one "new missile," and Moscow previously declared that the SS24, a separate weapon, is its "new missile." Thus the SS25 would be a second "new missile." The Soviets insist, however, that the SS25 is a modification of an earlier weapon, the SS13, and therefore is permitted under SALT II.

*Soviet encoding, or encryption, of electronic data from ballistic missile tests in excess of that permitted by the SALT II treaty. The Soviets insist that the encoding they are doing is permitted.

Q: Has the administration just found out about the violations?

A: No. As long ago as January 1984, Reagan sent a public report to Congress citing these two actions as examples of Soviet noncompliance with arms control treaties.

The violations were also cited by Reagan last June when he decided to "go the extra mile" for arms control by continuing to observe SALT II numerical limits.

Reagan warned then that he might take further steps if Soviet violations were not corrected.

Q: What is the Soviet response to Reagan's announcement?

A: A Soviet government statement last Saturday called the U.S. charges of Soviet noncompliance with SALT II "unfounded from beginning to end." The statement said that as soon as the United States exceeds the numerical limits of SALT II -- or otherwise violates its "main provisions" -- the Soviet Union will feel itself free from any further obligations under the pact.

Q: What is the response of U.S. allies?

A: The European allies have sharply criticized Reagan's decision in a North Atlantic Treaty Organization (NATO) meeting last week and subsequent statements.

Q: Is this the end for SALT II?

A: Secretary of State George P. Shultz said Sunday that U.S. adherence to SALT II as a restraint on strategic arms programs "is over." Secretary of Defense Caspar W. Weinberger said last Wednesday the United States is "no longer bound" by SALT II.

But it is still possible that for "economic and military reasons" -- or because Congress forbids it -- the United States might not exceed the numerical limits of SALT II later this year as currently planned.