The generation number affected.
The number of garbage collections survived by objects in that generation, before promotion. If count is
nil, the function returns the current promotion count setting.
An integer specifying which segment of the generation is to be affected. The default is 0, meaning the lowest segment of the generation.
set-promotion-countis deprecated, because experience has shown that it is not useful.
set-promotion-countis implemented only in 32-bit LispWorks. It is not relevant to the Memory Management API in 64-bit implementations, wherein you may be able to achieve the effect with set-delay-promotion.
LispWorks User Guide and Reference Manual - 13 Feb 2015