A media set is two or more volumes where the active files sequence on one volume spans to another volume. A second volume could span onto a third volume, and the third volume could span onto a fourth. This group of volumes is referred to as a media set. If you append your backups to the end of an existing media set, the entire set could grow over time to consist of a significant number of volumes. It is also possible that the volumes at the beginning of the set have no active data, but since these are part of the set, these volumes are not usable.
BRMS provides a means for expired media volumes in a set to be removed from the set so that the expired volumes can be returned to the scratch pool for reuse. Expiring inactive volumes in media sets could help to minimize the number of volumes you need to maintain and track.
You can have BRMS expire volumes in your media set using the Expire media set volumes (parameter of the Start Maintenance for BRM (STRMNTBRM) command. The default value for this parameter is *NO, because it is not necessary to run this function as part of your daily maintenance. If you append your daily backups to active media, it is recommended that you specify *YES for the EXPSETMED parameter periodically.