A Free Pool acts as a reserve for scratch tapes and cannot be used for Backup.
Other Pools can take Media from there when they have none free left (and move them back when expired if Pool is configured that way)
A normal Pool with Appendable Policy enabled will take the first Media that is not full and write to it until it is. If more space is needed the Job will always take a new empty Tape to continue.
If you want a Job to preferably use a special set of Media you can use the Preallocation List.
Open the Backup-Specification and go to Destination, select the Device you want to use and select Properties.
Below the Pool-Selection you have the Prealloc List, there you can add which Tapes from that pool should be used in favour of others. If the Job won't fit the Media defined there it will continue with the usual Tape-selection process.