I created 2 tables (header and lines) and created a Foreign key relation between them on the lines table.
From within the header form I placed a menu item button referencing the lines form.
Clicking on the lines button it to launch the lines form, I create a lines record.
When I save the line it auto-creates the foreign key reference.
Basic fundamental stuff I'm sure but I'm just intrigued as to where this is documented and so I can understand other similar stuff that I might be missing out on.
I only realised this after looking at the example of VendPaymFee and VendPaymModeFee and realising that the foreign key wasn't being explicitly handled in either table or form.