It would be very useful to identify a "note type" (i.e. document type) which is defaulted on a purchase order, if the relevant note type already exists against the supplier account.
For example, a note of type "NOTE" is created against a supplier, this note contains specific shipping instructions for that supplier. When a PO is created the note is automatically copied against the purchase order header.

This would a replication of what is already available on the sales order side for customers. See Sales ledger parameters > General > Sales setup > Note type