Hi David,
DIGI parts in service orders are not relevant for supply planning. So will not see them as a demand. You also do not get any ATP info for DIGI parts. You have to first release the order to service execution, then you can do the confirmation. In the confirmation UI you can specify the logitics area, from which the part will be taken. After service confirmation the inventory is then automatically reduced by the quantity of the DIGI part.