If “foreign_table” is enabled:
If set, then values which are not integer ids will be allowed. May be needed if you use itemsProcFunc or just enter additional items in the items array to produce some string-value elements for the list.
Notice: If you mix non-database relations with database relations like this, DO NOT use integers for values and DO NOT use “_” (underscore) in values either!
Notice: Will not work if you also use “MM” relations!