0 for the most recent generation, 1 for the most recent two generations, and so on up to a maximum (usually 3). Numbers outside this range signal an error.
Total free space in the generation.
Amount of free space in the generation which is available in blocks of 512 bytes or larger.
Amount of free space in the generation which is available in blocks of 4096 bytes or larger.
check-fragmentation is implemented only in 32-bit LispWorks. It is not relevant to the Memory Management API in 64-bit implementations, where gen-num-segments-fragmentation-state is available instead.
LispWorks User Guide and Reference Manual - 20 Sep 2017