This form contains a very interesting feature of ForX. The Interests checkboxes are grouped in a group called interests. This group must have one member checked to be valid by setting forx:grouprequires to any. The last textfield gets a special treatment, as it is only required when the Other checkbox is checked. This behavior is expressed in a conditional requirement expression:
forx:required="if other=='CHECKED'"
other is the forx:id of the Other checkbox.
Please note, that the textfield does not belong to the group.
This form also contains immediate checked elements, these are validated right after the user entered some data.