In this window you can map empty instrument slots to existing instruments. This allows you to have a fully functional soundbank in a small size. For example, if you have an Acoustic Grand piano in your DLS file, but no other piano-type instruments, once you've mapped instrument slots 1, 2, and 3 to instrument slot 0 (Acoustic Grand), any time a MIDI file needs to play instrument 1,2 or 3, you will hear Acoustic Grand.
Instrument slots in this window show one of three states: Real, Aliased, or Empty. You can select more than one instrument slot at a time to assign one Real instrument to many slots. If you right-click an Aliased or Empty instrument, you'll see a menu that allows you to indicate the desired bank numbers and instrument names. Select the instrument of your choice to set the alias.