A foreign key links two tables T1 and T2 by assigning fields of table T1 to the primary key fields of table T2.
Table T2 is then known as the check table of the foreign key.
Sometimes when you define a domain, you already know that all fields that use the domain will need to be checked against a particular table. You can store this information in the domain definition by specifying a value table.
If you try to define a foreign key for a field that points to this domain, the value table of the domain is proposed as the check table for the foreign key.