As well as PKHs answer
you need to be aware that media sets cannot be used to control exactly which tape a job uses - media sets in fact are only there to group tapes into groups of the same overwrite and append controls (which does relate to indivdiual jobs because it is data retention control).
However an overwritable tape in a specific media set can be moved by Backup Exec to another media set if it is the only tape available (based on rules that are covered in the tape handling section of the admin guide)
With regards your problem if you want a specific job to always use a specific tape (or tapes) you will need to partition the slots in the library and then put your LTO7 tape(s) into a specific partition before sending the backup jobs to the partition (and yes initially leave the tapes in scratch as per PKH answer). Once you partition a library in this way it can cause job failures due to lack of suitable media in the specific partition as such there are pros and cons to using partitions and you have to be very aware of available media (and how the media set/overwrite protection and append periods all work)
Finally (and this is just in case you happen to find out about this), BE can use barcode rules to distinguish between tape technologies - but these are not for your requirement, as barcode rules are used when you have two, or more, different tape drive technologies (and a mixture of tapes) in one library and they are then used to stop incompatible tapes being selected when the wrong drive is available (i.e use of an LTO-7 tape in an LTO-5 drive etc) As you have only LTO-7 drives, barcode rules won't help you.
