The Full Form of POH is Periodic overhauls.

Periodic overhaul is maintenance methodology to prevent a major unplanned breakdown where, based on fault or interruption history, a piece of equipment, or subsystem of the equipment, is proactively taken out of service and disassembled, repaired, parts replaced, reassembled, and then returned to service.

Overhauling a machine is fundamentally an economic decision. I don’t overhaul my pickup truck engine at 100 K, although I could. The cost-benefit analysis may convince me that a new truck is warranted. The need to “overhaul” equipment is usually based on the probability the machine will catastrophically fail in a manner that will require extensive repair and the event will be costly. Timing of overhauls (when to do them) is a major factor. Summer shutdown is used for these to avoid interruptions to production output.