RB modifier seems to do the "finesse" shot/pass, or makes the guy kick it off the instep. regular (no RB) seems to go off the laces.

so when you're dribbling down the line and don't have time or space to turn it back in towards the goal, RB+cross is probably going to give the best result, a nice curling cross to a teammate.

but if you can put a fake on the defender and face in towards the goal, regular cross seems to get the best results.