Ann,
        At installation time, ADSM defines the device DISK and that's the device
your policies are using.  First you need to define the device TAPE (or the
one you may already have) and assign it to a backup mgtclass.
Now you have two options here : backup directly to tape (everything) or
send only the biggest files to tape while the small ones may be sent still
to disk.

For the first option you need to change your mgtclass for the files you
want to send to tape, and for the second one you have a parameter to decide
whether you allow the file to be backed up by that stgpool or not, and
therefore you need to define a secondary storage pool (usually a tape
device) and assign it to the first stgpool.
                Hope it helps....Oscar Omar Ortega