These are the crucial features of the balanced-budget plan agreed upon Friday by key House and Senate negotiators:
--Deficit ceilings would be set at $171.9 billion for the current fiscal year; $144 billion for fiscal 1987, which begins next Oct. 1; $108 billion for fiscal 1988; $72 billion for fiscal 1989; $36 billion for fiscal 1990; and zero for fiscal 1991.
--If Congress failed to adopt a budget that meets the deficit target for any year, a schedule of wide-ranging spending cuts would automatically go into effect.
--The automatic spending cuts would fall equally on defense and non-defense programs.
--On the non-defense side, Social Security, certain benefit programs for the poor (including Aid to Families with Dependent Children, Medicaid and food stamps) and veterans' compensation and pensions would be exempt.