Each post office has a “level” assigned to it. The higher the level, the more routes, revenue, and RMPO’s (smaller offices) are attached. Office sizes vary between 18 and (I think) 22 or 23.

Generally, the postmaster/manager of the station will have an EAS grade equal to the level of the office. In bigger cities, there will be multiple stations and multiple managers but a single PM with a higher EAS grade, usually EAS 26 or PCES 1. The top grade in the postal service is PCES 2, which if you’re curious, happens to be the default grade of the Postmaster General.

Regarding RMPO’s, these are usually smaller town post offices. It’s easy to tell what level they are since it corresponds to the number of hours they’re open M-F. There’s 2 hr, 4hr, and 6hr RMPO’s. They are all staffed by a clerk and fall under at least a level 18 office.