Introduced in build 6.1, and enabled by default, persistent ABDs is a significant change from existing Alike ABD lifecycle.
Prior to 6.1, ABDs are always spun up and torn down per VM being backed up, meaning each backup target gets a "dedicated" ABD whose lifetime is always equal to the duration of the data acquisition phase of the backup.
After 6.2, the A2 will default to a new behavior. At the time of the VM backup, if all ABDs are already in use for the XenServer host are already in use, or none exist yet, a new ABD will be spawned. But after the data acquisition completes, the ABD will not be powered off and deleted. Instead, the ABD will remain running.
The next time a backup starts in the same host, this pre-existing ABD will be re-used.
Thus, in the normal course of Alike backups, ABDs will be created and left running on your XenServer environment. They will remain running indefinitely, until either the A2 is restarted, or a manual step is taken to reclaim them.
This has shown to dramatically reduce load on larger environments due to a "boot storm" of many ABDs starting up as nightly jobs kick off or any job with concurrent backups begins.
This also shaves a minute or two off the length of a backup, and for busy environments with hundreds of VMs to protect, a minute for each VM adds up to a big increase in effective throughput.
However, in environments where it is not desirable for ABDs to be running outside of the backup window, this feature can be disabled.
You can disable Persistent ABDs under Tools->Settings->Advanced.