Diagnosis
You are trying to select HUs which do not have a reference and thencreate a reference for these HUs for the selected document (request,order, delivery).

Procedure
Select just one header row. For the material for this request item,order item or delivery item, HUs without a reference are selected. Inthe following list display you can then select for which HUs you want tocreate a reference. To do this, choose Copy or Transfer HUs. Thecorresponding reference is then created for these HUs.