A UoM (Unit of Measurement) group is a set of UoMs that you want to use for a certain type of product. Each UoM group has a base UoM. All the other UoMs belonging to this group are related to this base UoM by conversion rules.
You can assign a UoM group to an item. When you use different UoMs for the item in sales, purchasing, inventory, and production transactions, SAP Business One automatically processes the differences in UoMs according to their conversion rules.
Source table: OUGP.