A scheduled Backup can be full, incremental, trans or whatever mode you desire it to be.
This changes nothing in the way the Medias are allocated. This is defined for each Pool separately.
If a Backup (of any mode) is started it searches for the first unused Media in the defined Pool that has space left to write onto. Once this Tape is written to the end a empty Tape will be used to write the remaining data. This to avoid unrequired fragmentation on Media.