Worked with it in a few scrimmages. A handles the ball at all times until foul enforcement when U marks off. U gets ball after play outside numbers and relays to A (much like he might do to the R) A keeps ball from being snapped on substitutions. A has responsibility for the L's tackle (allows L to drift down into the belt). R only trails runner to A's side if it's the QB. A is with the kicker and remains in the middle of the field.
Probably more, but that's what I remember.
