Sorry I completely missed this post. Here’s the answer just in case:
The purpose of the Device ID Parameter in the Permit Join API is to identify which interface you want to enable the permit join for.
For example: Lets say you have 1 Zigbee USB-CID and 1 Insteon PLM attached to your RaspberryPi. You can send the Permit Join with the ID of the Insteon PLM and only the Insteon PLM will be in open join mode (start linking mode). The Zigbee USB-CID will not be affected.
Hope that answers the question.