Call Park is the ability to place a call on hold into a specific parking location (a fictional extension) such that the call can then be picked up by another extension. This can be used where a call can be parked, the called party can then call several other numbers and when the desired party is found, they can simply pick up the parked call by dialing the parking extension number.

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so that you can access the call at that extension by selecting the line key that is on hold to retrieve the call.
(This information taken from a Redwood PBX)