The ST_USE_FIXMEM constant determines whether functions for fixed-size memory block allocation are included or excluded from compilation. Functions will be compiled if the constant is set to 1, and will not be compiled if the value is set to 0.

If this constant is not defined, it defaults to 1, causing the functions to be included in the output code. If fixed-size memory management functions are not required, they can be excluded from compilation by setting ST_USE_FIXMEM to 0, thereby reducing the output code size.