This has been assigned, but it is not yet implemented (as of 1.1.2pr2).
Some of the stylesheets used by devgui use the information about foreign keys to create automatically tags allowing to select from a referenced table. In the above example, we might get a:
<db:select fieldName="cust_no"> <db:tableData name="customer_in_order" foreignTable="customer" visibleFields="first_name,last_name,phone" format="%s %s (Phone %s)" storeField="cno" /> </db:select>