Performing Archive rather than Backup makes more sense for this
purpose, as it's intended for such exceptional put-away needs.
Consider defining a separate management class and storage pool, to
best identify such deeds.  If you must go with Backup, I'd proceed as
below...

In most cases, a tape sent away in this manner calls for a retention
period that is different than the usual, in that it's pointless to
have a tape nicely secured somewhere and yet have the files on it
logically expire.  I would thus set up a separate management class and
backup copy group for this purpose, making the management class name
descriptive.

I'd also define a separate storage pool for it, with a MAXSCRatch
value of 0, and define an empty tape into it.  This will allow you to
choose a tape and assure that it will contain only the desired data.
Then set up a temporary Include for that management class, perform the
special backup, and checkout the tape, marking it Offsite (to prevent
inadvertent mount attempts from waiting on it).  Remove the temporary
Include.

Separate definitions allow you to readily identify such separated
data, when you have to look back some months or years in the future.

   Richard Sims