A model’s Command Distance determines how close it needs to be to other friendly models in order to not be considered alone. An alone model behaves as if it is one training level lower than usual (this does not apply to badass models -they are damn unshakeable).
Command Distance also determines the level of influence a better trained soldier can have on those who are less well trained. If a model is within their own command distance of a better trained model, they can use the higher courage rating to take any courage tests.
This rule is yet to be tested in anger, so is subject to a great deal of change.