Custom fields apply to one subsidiary ledger (such as an inventory item) or one form (such as a purchase invoice). The only linkage that exists is when copying one form to another. If identically named custom fields exist on both forms, the content will copy.
If I understand correctly what you are trying to do, you should define a custom field for your inventory items for the supplier stock code. Set it to show as a column (if desired) and on printed documents. Whenever you buy that inventory item, the custom field will automatically appear:
It will also appear on sales invoices, credit notes, etc., any time you use that inventory item. So two things need to be true:
- You always buy from the same supplier, and
- You want the supplier’s code to show on all transactions, no matter who sees them.