A column in Sunday's Business section incorrectly listed the address of the Bureau of the Public Debt. It is 1300 C St. SW.

President Reagan's budget, under attack on Capitol Hill as underestimating defense spending by as much as $15 billion, might be understating the true cost of national defense by many billions of dollars more, according to some analysts.

The real defense budget, they say, is a matter of definition.

The budget's so-called national defense function provides the most commonly cited number. This category -- totaling $282.2 billion in outlays, or spending, in fiscal 1987 -- covers most of the Defense Department, the Energy Department's atomic weapons programs and several small "defense-related activities," including civil defense and the Selective Service System. It represents a 28 percent slice of the budget pie.

"We think the defense function accurately defines the defense portion of the budget," said Edwin L. Dale Jr., spokesman for the Office of Management and Budget. Any other definition, he said, would be "arbitrary and subjective."

The national defense function also defines defense spending for purposes of the Gramm-Rudman-Hollings deficit-reduction law, which stipulates that 50 percent of the target reductions should come from defense spending.

And it generates the defense spending figure that the Congressional Budget Office, as well as the chairmen of the House and Senate budget committees, have criticized as being as much as $15 billion too low in light of Pentagon spending plans.

From other perspectives, however, the budget includes many mislabeled or hidden defense expenditures, large and small, that by tradition -- or because of political decisions -- are counted in other functional categories. In some cases, the defense role of these programs is specified in the budget text. In others, it is plainly implied.

Veterans programs comprise a separate budget function worth $26.4 billion in outlays in fiscal 1987. "Federal benefits and services for veterans and their survivors recognize the sacrifices that wartime and peacetime veterans made in military service," the budget states.

These costs of national defense past include pensions, disability payments and medical care.

A larger category of expense is interest on the public debt, totaling $148 billion in 1987 outlays. Stephen Daggett, a senior research analyst at the nonprofit Center for Defense Information, a Pentagon watchdog group headed by two retired admirals, attributed 66 percent of this function to defense in a review of the 1986 budget.

He said the figure -- derived from a computer analysis of military spending, federal funding and deficits dating back to 1940 -- would be "in the same ballpark" for 1987.

Another category that includes hidden defense expenditures is international affairs, with 1987 outlays of $18.6 billion. "Expenditures proposed in this budget support U.S. foreign policy and national security objectives in these difficult times," the budget states.

Under this function are $10.9 billion in international security assistance programs, which the budget calls "vital instruments in the exercise of national security" that help ensure "U.S. access to military bases and facilities overseas."

A different subcategory worth $3.1 billion is primarily for State Department operations, but also covers activities such as the Arms Control and Disarmament Agency. It seeks an increase to protect U.S. employes and foreign diplomats against terrorism.

Under transportation, a function that totals $25.5 billion, the budget says the federal government "seeks to facilitate a safe and efficient national transportation system that," among other things, "supports the national defense."

The Coast Guard, a $2.6 billion agency that becomes part of the Navy during war, will continue to modernize its fleet, the budget says, "resulting in expanded law enforcement and defense preparedness capabilities."

Merchant marine subsidies and the civilian air traffic control and weather radar systems also contribute to national security, but more directly supportive is transportation research and development by the National Aeronautics and Space Administration, including its partnership with the Defense Department on hypersonic flight technologies.

NASA also figures under the $9.2 billion general science, space and technology budget category. The space agency's defense role, however, has traditionally been difficult to quantify.

A 1984 comptroller general analysis for Sen. William Proxmire (D-Wis.), which covered most of NASA's fiscal 1985 budget, concluded that at least 10 percent reflected direct Defense Department support and another 9 percent reflected mixed military and civilian concerns, an aide said.

Other science activities with potential defense applications include the Energy Department's research in high energy and nuclear physics. Science programs "help to ensure U.S. strength and leadership in science and space technology," the budget says.

Defense support also is found under the $4 billion energy function. Research and development in nuclear fission, the budget says, is being redirected toward "reactor concepts that can meet space and military nuclear power requirements."

This function also includes the emergency energy preparedness program, with an allocation for the stragetic petroleum reserve.

Benefits for civil service workers, retirees and their families -- the Pentagon employs about one-third of the civilian federal work force -- are spread around budget categories, from health (insurance benefits for retirees) and education (impact aid to school districts) to income security (retirement).

Andrew A. Feinstein, staff director of the House civil service subcommittee, estimates that $4.3 billion in outlays not covered by the Pentagon budget are attributable to Defense Department civilian retirees, because employe-employer contributions do not cover the full cost of the retirement system.

Another budget category, the $6.9 billion justice function, includes not only courts and federal prosecutors involved with defense procurement cases, but border enforcement, the Secret Service and the Federal Bureau of Investigation. The budget seeks additional funds "to increase the FBI's foreign counterintelligence activities."

Finally, the federal bureaucracy has overhead expenses, accounted for in the $6.1 billion general government function. This category covers "overall management, policy and central operations," such as tax collection, personnel policy, housekeeping and recordkeeping. Business accounting would allocate a share of these costs to defense.

General government also includes Congress, which authorizes, funds and investigates (through the General Accounting Office) defense programs.

And this category covers White House expenses, from the National Security Council and science office (a booster of the Strategic Defense Initiative) to the commander-in-chief's $250,000 salary.

In all, an inclusive national defense budget would appear as a much larger share of the fiscal pie -- half again as much as the official 28 percent, if not more. In the final accounting, the size of the real defense slice remains a matter of definition.