OK - if you have the license then great!
So you need a volume (not just a directory / folder) as the disk source, so a dedicated disk volume to use - you create an Enterprise Disk using that volume (The Storage Server wizard will take you though creating the Storage Server (media server that uses Enterprise Disk) - follows on to make the Enterprise Disk Volume - follows on to making the Storage Unit for it.
Next you create one or more Storage Lifecycle Policies. In that you add a backup destination ... the disk Storage unit just made and set the retention you would like it to have on disk (better still use capacity based so that it keeps as much as possible, at least up to the retention period.
You then also add a duplication destination .. your tape library and specify the tape storage unit and tape volume pool plus the retention on tape
You may want to have Daily, Weekly and Monthly SLP's to cater for all different retention periods - the beauty is that they can all go to the same disk storage unit.
You then set the SLP as the destination storage unit in the Policy.
At this point retention periods in your schedules cease to have any effect as it is all controlled by the SLP
So say you set the Weekly SLP in the policy attributes you would override that in the Daily Schedule to use the Daily SLP and the Monthly Schedule to use the Monthly SLP
When a backup runs it will go straight to the backup destination specified in the SLP using the specified retention ... as soon as the backup completes the backup gets duplicated to tape, using the pool and retentiuon specified in the SLP
All very efficient
Hope this explains things