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).