Any physically empty HU is reported using a distinct transaction. That means that any residual book inventory quantity that may exist in the ERP system, assigned to the shop floor and to that specific HU is reported as consumed and issued to a distinct cost center. Inversely, should a workplace report a consumption from a HU that has no book inventory in the shop-floor stock (is already empty in the ERP system), an error is generated as a backlog in issuing the consumed materials, see chapter "Meaning of backflushing". What kind of unplanned transactions are used to handle this error is a question of internal policy of every firm. The reason why residual book inventory may exist in the MRP/ERP system may have multiple causes. A few of them are: - the quantity in the original packaging was not completely exact (e.g. the supplier indicated a quantity of 10000 meters per coil, but some coils had only 9500 meters); - the workplace has consumed some materials for adjusting the machine without being able to report that quantity exactly; - the operator has omitted to assign a new handling unit to the workplace or has assigned the false handling unit. Such errors can be avoided if the system has a good data validation. For example, the machine does not start processing the job until all handling units required for that job are assigned to the work place or machine. It is very important to design the interface between the MES and the MRP\ERP properly: the reporting of the consumed quantities has to be processed consistently, in the right order, in other words all the recorded/reported consumptions from a handling unit should be issued in the ERP before issuing the handling unit as empty. Otherwise a large number of false errors would get into the error-backlog because the book inventory is already zero as a consequence of issuing the handling unit as being empty before issuing all the consumption which refer to that specific handling unit. These two alternatives to backflushing based on the pull-strategy (preproduction issuing/postproduction issuing) have also a self-correcting property: real quantities are reported from each workplace (not based on the bill of materials) and each unitized packaging (handling unit), when empty, is reported as empty and any residual book-inventory on the shop-floor is discarded.