userbldg.p will take the current user (as defined in common.i) and see if they are allowed to work in that building. If so, then TRUE is returned.
The way this works is that if they have no user-bldg record for this building, but they have entries for any other buildings, they are not allowed into the building. If they have absolutely no user-bldg records or the have one for this building, they are allowed into the building.