Monica Rathbun explains a few instance-level backup properties:
Default backup media retention in days. Now the first things that comes to my mind is that “hey this is a cleanup job” SCORE! Thinking that maybe this will auto delete old backups. After all isn’t that what retention means? NOPE, not in this case.
In this case it’s just a number of days before that a backup media can be OVERWRITTEN. If the DBA goes to overwrite the media before those days it will give a warning message. You’ll note in every back up action you do the RETAINDAYS option is filled in. In this case it will always reflect to 90 now that we have changed it. In general, this a pointless option to me. I don’t normally OVERWRITE backup media. To me this was more relevant when Tapes were used and disk were harder to come by, so I leave it alone.
Read on for more settings.