Its generated off of the future sales forecasts and the assumption of a certain productivity number that generates for your store. The productivity number actually creates incentive for the STL to spend every single hour they can (without going over) because it tells corporate you needed all those hours to run your building successfully. It therefore means you are "less productive" and will try and keep that payroll for the following years. If you go a long time without using all your hours your productivity is very good then, and you "could" run on less hours as a whole.
It then spits out a total hours for your store based on the weekly/monthly sales. The total bucket is then divided by workcenters depending on each individual formula (so flow is based on trucks and average carton count, front end is transactions and units rang etc). The fixed workcenters will receive their total no matter what happens and the flexible workcenters will receive the remained (basically it figures their average workload between hardlines, softlines, market and scales it evenly between). There are a few other specific factors at play but that is a very basic idea.
The problem is that most STLs just send out their own excel sheet with their own allocations anyway, so the entire system doesn't matter except for the total hours you receive each week based on the sales.