Surprisingly I've stumbled upon a widget whose type was expression. I've never heard of such widgets before. According to LIST-QUERY-ATTRS it has the following attributes:
As I got in order to create such a widget you have to use FORM HEADER statement and all widgets which are normally fill-ins become expressions. The latter don't have any attribute to get their screen values though. That was quite an unexpected surprise. Moreover the attempt to query this attribute using CAN-QUERY leads to 4044 error. I hope I'm wrong about that. Does anybody know how to get their SCREEN-VALUEs?