The variable is the position of the current call in the queue that it is in.
If the call is not in a queue it has no value. For a call that is in a queue the following will return True if this call is in position 2

If you are wanting to offer a call back and are using the "Test Condition of Queue" see doc here this will return an outcome of the test but it does not fill the $queuePosition variable.
You can do this while not yet in a queue or you could even be testing a different queue. You may even want to test before putting the caller into a queue.
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Oisin Glynn
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