The Reader Zone ID property is used to assign a location ID to the reader which indicates what side of a door it is on and what room(s) or area it is granting access to.  This can be confusing since a reader with zone ID 27 may be physically located in zone 26, but it is controlling access to zone 27.  When a user is granted access the zone ID is recorded in the logs to show where that person last gained entry.  This can be very useful for determining if a user is still in a specific area (e.g. in the building).  

Readers located on the outside of a building should be programmed with a zone ID of the area on the other side of the door.  Readers located at exits should be programmed with a zone ID of zero (0) to indicate the user has left (or intends to leave) the building.  There is no guarantee that the user, once granted access actually passed through the door.

See the example below... a user gains access at the main entrance to the lobby - it is programmed for zone 1. The user is now expected to be in zone 1 - the lobby.  Next they gain access to the warehouse via the lobby door.  That reader is programmed for zone 4 - the warehouse.  Once they pass into that zone they are expected to be in zone 4 - the warehouse... and so on.  If they use their token (RFID card) to leave via a loading door, equipment room or the lobby, those readers will be programmed with zone 0 - outside.  So in general the reader is programmed with the zone it allows access to - or what's on the other side of the barrier.