UOM Name not coming in AP Invoice from Item Master data for Item with Manual Unit group

Hi,

I have created an Item and assigned it the UoM group Manual.

In the Item Master->Inventory Data tab, I have entered the UoM name, but it does not come in the UoM Name column in A/P Invoice. Moreover, it allows me to type any arbitrary name in this field.

What is the use of entering any arbitrary name in the UoM name field in AP Invoice, and why does the system not pick the value entered in the Item Master?

Thanks.