BM and IM transactions are always in Standard UoM.  Sales is the default UoM in SO / SO invoices.  Be very careful with this feature (especially with generic terms like "BOX" or "CASE", which can vary from item to item).  When we use UoM conversion, we often have to modify forms (eg. picking sheets) to show the conversion factor, for users to clearly see what the quantity means on each line.