Hi,
In ME51N/ ME52N, you can assign source of supply for individual PR line items.
ME56 (RM06BZ10) is for assigning source of supply collectively, for example, you can select the PR's for same material / material group / MRP controller etc and then assign the source collectively. System will automatically assign the source of supply based on the source of supply options (like contract, info record etc) available.
Regards,
AKPT