Customer have reviewed MIM PAGEFIX description on Programming Guide.
Typically, the CA MIM for z/OS working set size remains fixed in real storage and there is no need to page fix its cell pool resident control blocks. However, in times of critical pageable storage shortages, frames can be stolen from the working set pages. If a given system is configured as such that it is real storage constrained, then you may want to prevent real frames from being taken from the CA MII address space.
They asked what exactly "in times of critical pageable storage shortages" is.