A paired, primary-secondary relationship indicates the door strike and door contact will be shared between two readers when the readers are used to control the same door. An example is when a door is secured for both entry and exit access. In this case, a reader is located on the outside as well as the inside of the door.
The primary reader controls and reports all door settings (Door switch (including contact), Lock settings, REX, and automations) meaning that if a door is forced open, the alarm will be reported for the reader selected as primary. However, the secondary reader can be configured to be Enabled during schedule. If it is not enabled for this, the secondary behaves as if it is set to "Always" and follows the primary's setting.
Door mode changes can only be done by the primary reader. (See Set door mode.) However, the door can be temporarily unlocked by either paired reader.