If fixed-size is not
nil, it tells LispWorks that it should keep this number of bytes of reserved memory. The reserved memory is held in segments of fixed size of 8 MB, the actual amount reserved is fixed-size rounded up to the nearest 8 MB. If fixed-size is
nil, LispWorks uses the total size of generation 0 plus generation 1 after each GC to compute the size to keep.
set-reserved-memory-policy does not change the current size of the reserved area. LispWorks only reduces the reserved area after a GC if it is too large. It only increases it, up to the limit, when the GC has copied all objects from a segment.
LispWorks User Guide and Reference Manual - 20 Sep 2017