I built a query some time ago and had to modify it the other day. I had difficulty figuring out which fields I used as some of them are very similar or generic and it took some time to find the exact category where this field came from. The program should highlight and/or select the used fields in the Available Fields Pane. This way it is very easy to see what fields have been used already.

This functionality was already implemented in the 'FAST' module of Accounts Payable in FE.