USB to main is a boost-by-dB function for the oncoming audio signal via USB (from the computer to the Digitakt). I think it can be used be for leveling DAW master volume with Digitakt master volume when you are using both side-by-side, or lowering sample volume when sampling USB etc.
On the other hand, Int to main allows you to turn off the Digitakt local audio if you’re routing/monitoring the Digitakt trough your DAW. That way you don’t get duplicate signal (DAW Digitakt + Local Digitakt) at the same time.
Pre-fader / Post-Fader function allows to select wich of those two configs you want to use for send fx channels input. I don’t remember clearly the concepts about those to be honest, but it’s related to proportions being send to the fx unit when you change the db values of that particular channel, or something like that…
PS: Sorry for the bad english hahaha