This is a convenience function which will extract the Apple App PID property from linked files. The property value is then saved to the Apple App PID UDTI.
The function is only applied to tracks currently linked and is equivalent to:
Get Property Linked PID -> Variable 1
Import value of User Defined Text Item with name 'Apple App PID' from Variable 1 (create)
The function immediately unlinks the tracks. This is to prevent the inadvertent overwriting of Apple App metadata when saving the tracks. If that was your intent you can relink the tracks before doing a Save. The unlinking protects the scenario where you have metadata in the iTunes library but little or none in the tracks.