Display / Proc.
Field which must be unique for all children of a parent record.
Say you have two tables, products, your parent table, and prices, your child table (products) can have
multiple prices. The prices table has a field called customer_group, which is a selector box. Now you want to be
able to specify prices for each customer group when you edit a product, but of course you don't want to specify
contradicting prices for one product (i.e. two different prices for the same customer_group). That's why you
foreign_unique to the field name "customer_group", to prevent that two prices for the
same customer group can be created for one product.