A field containing quantity amounts (data type QUAN) must be assigned to a reference table and a reference field. Explain?

As a reference table, a system table having all the valid quantity units is assigned or any other table, which have a field with the format or quantity units (data type UNIT).  This field is known as reference field.

The assignment of the field having quantity amounts to the reference field is made at runtime.  The value in the reference field verifies the quantity unit of the amount.