Hi Robert,
The substitution table is only used for Multiscreen shows where MiniMes are put together to make a single screen. When running in this mode, the default behavior for background sources is to zoom in on the sources so they fill all screens. If however you have a device that produces multiple video streams for a single large image, then you can group them together in the substitution table so the switcher takes them as a group.
I hope this helps. Let me know if you have any other questions.
Gary
#Carbonite