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 Enabled during schedule. See Add a reader.