How does Frequency Agility work in the EmberZNet PRO stack?

Frequency Agility essentially means a network channel change. The channel change process is governed by a Network Manager device (typically the Coordinator), which changes the channel as decided by the application in response to incoming channel quality notifications.

A node will automatically notify the network manager (NM) when enough of its unicasts fail. Per Zigbee specifications, a report will be sent to the NM if all of the following conditions are met:

There have been at least 20 attempted transmissions
One quarter or more have failed
It has been at least 15 minutes since the last report

For the NM, the process is generally as follows:

The child node's stack detects the unicast failure and scans and sends a notification to the network manager. This part happens automatically in the stack.
Your network manager receives the notification in its network manager.
NM decides whether to change channel and issue a network update.