There are 3 different key combos to activate recording precisely because there are 3 different sources. (ie Input AB, Input CD and Internal).

They are all the same method: press Track + the relevant source button.

During a performance, you might want to sample AB to Track 1, CD to Track 2 and the Cue output to Track 6 - all without going in to a menu.

It is a complex machine but, for the most part, it has an extremely well-designed interface.