I got it here: zigpy-xbee/zigpy_xbee/zigbee/application.py at d3dfb11b336e4bbaaaa0abe2d0b7627c6ac07f83 · zigpy/zigpy-xbee · GitHub
It is the default trusted center (usually, the coordinator node) link key documented in zigbee standard. It is used during join to transfer the actual network key, which is randomly generated by the trust center node. It is not used by nodes when they already joined.
You can. Please note however:
- The key must be preconfigured on all new nodes joining the network. If all your devices are xbee, then it is not a problem. For less configurable devices such as sensors it could be an issue.
- Currently the link key is hardcoded in the zigpy_xbee code. If you overwrite it, you will need to do it after every zigpy_xbee update.