Multi-programs, or multis, can load multiple sounds or programs and combine them.
You can use multi-programs to layer several programs or to create split sounds by setting several programs to the same MIDI input channel, for example. However, the most common usage is to create sound sets with different instruments set to individual MIDI channels.
A multi-program contains all plug-in parameters. If you use HALion as a plug-in in a Steinberg DAW, the multi-programs are listed in the Preset Management pop-up menu of the host application. You can drag multis and programs from the MediaBay to a slot in HALion.
If you use HALion as a plug-in in a different host application, you can use the preset functionality from the host application or the multi management features provided by HALion.