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After leaving office: a UPSC Chairman cannot take any further Union/State government job; a State Commission Chairman can only become UPSC Chairman/member or another State Commission's Chairman (nothing else); a UPSC member (non-Chairman) can only become UPSC Chairman or a State Commission Chairman (nothing else); a State Commission member (non-Chairman) can become a UPSC Chairman/member or any State Commission's Chairman (nothing else).
This creates strict "revolving door" restrictions preventing former Commission members from taking other government jobs afterward (beyond narrowly-specified further Commission roles) - removing any incentive for a sitting Commission member to make recruitment or disciplinary decisions with an eye toward currying favour for a future government job.
This is why a former UPSC Chairman cannot simply move into a plum government ministry role after their term - the only further roles they're eligible for are specifically listed alternative Commission positions, precisely to prevent any appearance of a government-job reward for favourable decisions while in office.