I've typed up too many long responses to this in the past and given too many detailed explanations to people at the store for everyone to just say "oh ok" and then blow it off, so I'll give the abbreviated version this time.
Long story short, the part of the system that keeps track of how much of everything gets sold (the "Accumulator") gets out of whack sometimes. Various reasons, various ways. Complicated (at least to simpler people). So eventually you get to a point where it wants more than will fit. So what you do is "reset the accumulator". You do this by "SUBT-9999" (this documentation can be found on Workbench - it's an official process). To do this, start of by backstocking as normal. Then, right away, do a guest pull (SUBT) on it. Put 9999 as the quantity. When it asks if you pulled it all, say no. It will ask you for how many are left. Obviously, you put how many you just STOed there. That's all. You have to make sure that you select "1. Guest Request" and not "2. Outdated/Damage." This is important. Option 1 affects the accumulator. Option 2 does not. Option 2 is useful for small moves that don't require a full LOCU.
To make the process faster, you can skip doing the SUBT-9999 process on items you are backstocking that already have locations in the stockroom. 95% of the time, it won't be asking for that item if you backstock it right now, because, obviously, it already has locations. If it wanted it, it would have asked for it already. Not 100%, but 95% of the time it does save you some time.
