Missing field for entering Internal Order Number in salesorder item

Hi Friends

We are trying to set up milestone-billing in debit-memo-request switching itemcategory from L2N to TAO. This woks fine except for one thing:

On TAO-items we are not able to assign an internal order number in (VBAP-AUFNR) like for L2N-items,

The field is invisible in item accounting view for TAO-items.

Anybody have an idea what controlls the display/hiding of the internal order number field ?