With an "A" service all filters are replaced along with the oil. With a "B" service, they just do a lube and inspection. There are variations on the theme, but is the gist of the two types.
Stop in at the next truckstop and ask them. If you ask a mechanic I think A service will cost more. If you ask a lot lizard I think B service will cost alot more and may not be legal in all states.