How is F4102.SNS used - Rounding Rule

I am wondering about the order of operation for alias SNS - Rounding Rule

By default it is available to populate in the item master record but not in the item branch. I understand that on a new item entry the value will be copied from IM to IB but if it is changed after the initial load, the two records are out of sync. Is the IB record ignored or does it look at IB and if there is no entry it goes to the IM record. If the IB entry is not used at all, how does one item branch manage it different from another item branch?