Media pool is more about cassette storage, while, media set is a portion of it. There is a good post
that describes it in detail; so, kindly take a look.
As far as I can understand, you’re more after “vault” option or, in other words, the concept of offsite place to where the tapes that hold backup data are taken. If so, please be aware, that there is no such functionality in the current product version, so, you should, probably, use media “description” field as the place where the name of corresponding vault can be written.
However, according to a plan, the tape vault reporting staff should be present in one of the future releases.