Hi Jan,
there's also some apparently widespread misunderstanding regarding medium export. As Bob already explained, export is another (and IMO not very appropriate) name for "remove this medium from the IDB". Why would you do that? Except for some media maintenance operations (getting rid of defective media, dealing with media moving between different cell managers etc), there is no reason to ever export media. Specifically, export is not the natural operation to chose when vaulting media offsite. You may move it to another location, but that's optional either.
BTW, DP allows you to set catalog protection larger than data protection, which is very helpful in this context. You normally don't want to lose catalog the very next noon after a medium expires, given it may live on for days or weeks (maybe months, if vaulted) without getting overwritten. There was some IDB scalability issue lurking in there which lead people to cap catalog protections and even to export media just to maintain IDB size and resonsiveness, but that is hopefully a thing of the past with the new IDB in 8.0+.
HTH,
Andre.