This article explains what the archive purge option means for the archive sets on your media.
If there is not enough space available on the archive media for the incoming set, purge deletes the archive sets that have exceeded their retention settings to create space for the new set. For more details about archive purging, see Purge in the Recovery-Series and UEB Administrator's Guide.
If there is an archive set outside retention, but it contains a backup for a client that is considered a dependent in another archive set within retention, that older archive set will not be purged until the newer one is also out of retention.
Purge (or "Delete older backup data to free space" in the new UI) is applied to the archive set at the time the archive job runs. It cannot be changed later on an existing archive set. Any archive set created without Purge or Overwrite enabled (or "Fail backup copy job and send alert" in the new UI) will not delete older jobs, even if it is out its retention.