Slitherine

Hello, I see that in impact you can have a support fire from bows if behind MF or HF stationary.
What does it mean? Bows must be in the same BG of supported foot? Not possible if they are 2 different BGs just one behind other?
While creating mixed BGs, i can only if army lists allows me mixed BG of 2/3 and 1/3, or can i build mixed BGs of any type of the same quality and training?

The support fire is only allowed from mixed battle groups. Yes, these are only the ones permitted in some lists.

or BGs who are unmixed but all have weapons that can support fire - e.g. Bows