Options for Detecting a List Transition

You can detect list transitions two different ways:

By having your list notification exit receive control when the list changes from empty to nonempty. Your list notification exit then invokes the IXLVECTR macro to check the state (empty or nonempty) of each list you are monitoring.
By coding a polling routine to invoke the IXLVECTR macro periodically to check the state of each list you are monitoring.

For each list you monitor, you can choose how you want to detect list transition. You can monitor some lists using a list notification exit and others by whatever method you choose, such as polling the list notification vector.