It's just an internal system thing.
For whatever reason in the original design, if you want to charge the battery to a figure that is less than 100% then you need this set to 1 - otherwise it flat out ignores whatever % you've told it and charges to 100% regardless.
Caught a few of us out in the early days when we were messing around with the API stuff.
It's of no use to the normal user portals, nor if you set things via the API (as it already handles all this flag setting for you) -
If you want to see it working you can adjust your "Charge to" % and read the register but that's all. It's not a secret battery heater or anything I'm afraid!