Exactly. All you're changing is the accumulator value for the DPCI. You've only actually backstocked 1 item.
The accumulator tracks each DPCI and has a trigger that adds the item to the CAF batches once enough has sold through. Let's use a box of Kleenex as an example(I am making up values):
The accumulator starts at zero and the trigger is say... 5. Every sale of this particular Kleenex raises the accumulator. Once it hits 5 the accumulator tells you in the backroom to pull 5. Replenishment in it's purest, ideal form. If instocks comes through and scans the item in outs, that automatically sets the accumulator to whatever the capacity is. Lets go with 25. So now it wants to pull 25 from the back, but wait! You only have a carton of 10. So those 10 pull and go to the floor. Next time that Kleenex comes in on the truck we still need 15(assuming we haven't sold any), they will mark 2 cartons(20 eaches) push and any after that will be sent straight to backstock. Those 15 fill the floor and you backstock the 5 extra in wacos. The accumulator is now sitting at 0 again because we have filled the floor successfully.
Subt9999 is like a Jedi Mind Trick. *waves hand* These aren't the Kleenex you're looking for. And just like that, the system knows the floor is full. It can't decrease below zero. So in our example above the system wants 25 Kleenex. Subtracting 9999 from 25 will make it 0 again.