The function set-delay-promotion
delays promotion for generation gen-num, which means that objects are promoted to the next generation in the second garbage collection (GC) that they survive in generation gen-num. By default, objects are promoted in the first GC.
It is not obvious under what circumstances delayed promotion is more useful than the default behavior. If you find this function useful, please let us know at Lisp Support.
set-delay-promotion
is implemented only in 64-bit LispWorks. It does nothing in the Mobile GC and its return value is not meaningful. It is not relevant to the Memory Management API in 32-bit implementations.
LispWorks User Guide and Reference Manual - 20 Sep 2017