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You define the Overwrite protection period and the append period on a media set which nothing but a logical container which protects you tape . By default all the new tapes are in Sctrach media set . When you run the 1st backup job the tape is picked up from the the scratch media set and is associated to the media set define in the backup job properties .
Later when you run the overwrite job ,depending on the settings you have under tools--Option media mangement -- it would either select the recyclable tape (whoes opp is expired) or a scratch media .
You can specify if it should overwrtie scratch media 1st or recyclable media .
Refer the following document