Quantity unit refers to the physical quantity in which an item can be counted or measured. In many cases, the quantity is specified in the units “piece” or “piece per packaging unit”. For small-volume items or bulk goods, weights or “pieces/kg” are also common.
Cases that are difficult for inventory management include

if a change of unit takes place between goods receipt and goods issue (e.g. a change from meter to piece when making up hoses or a change from unit area to piece or weight) or
when the number of pieces per packaging unit (sales unit, outer packaging unit, etc.) changes.

Source: logipedia / Fraunhofer IML