A button set to this action can be used to park and unpark calls. The button can be set with or without a park slot number.

When associated with a specific park slot number, the button can park and unpark a call from that park slot. When a call has been parked in that park slot, the button's lamp flashes. Green if the call was parked using that button, red if parked by another method. Pressing the button again retrieves the call.
When not associated with a specific park slot number:
If pressed while you have a call connected, that call is parked using a park slot number assigned by the system based on your extension number.
If pressed while you have no call connected, your phone displays details of your parked calls and allow you to unpark a selected call.
When you have calls that you parked (other than Park & Page) or others parked against your extension, the button's lamp flashes.