When a purchase line is created the associated nominal code comes from one of the following sources and in that order:-
- Supplier value for default.
- Material record.
- System value for default.
The relevant default (see the Processes+Show defaults menu) is in the MC module and called Material Buying Nominal Code.
So for example, to always associate a particular nominal code with a supplier (regardless of the material) create an instance of the default for that supplier.
You can do that by going into the detail of the supplier, clicking the Defaults button and then creating a supplier instance with the New button. The Selected Default will be as above. Set the value as required