One of the two unit of measure modes you can select when you turn unit of measure on.
When you select Single U/M Per Item as your unit of measure mode, you can assign only one unit of measure for each item. That single unit is then used for that item everywhere in QuickBooks. For example, if you have an inventory item called "rope" and you assign it a unit of measure called "foot," then that item must be purchased by the foot, counted in inventory by the foot, and sold by the foot.
Unit of measure set (definition)
Change the unit of measure mode
Multiple unit of measure per item mode