@Stuck_in_an_ATR:
FOT (Flight Operation Telex) 999.0076/09 refers.
BUSS activation comes with increased workload (manual flight, manual cabin press control, some FMS functions not available)
Once you select BUSS, it is for the remainder of flight because you won't be able to identify the faulty source anymore.
Agree the BUSS is not compensated for Mach (as it is not compensated for speedbrake) but that is not the reason for the FL250 threshold.