I want to set up two custom fields where one field provides the lookup reference for the second. E.g. I have a category called 'lighting' in Field 1 (called System), which when selected, produces a selection list in Field 2 (Sub-System) that presents the choice of 'internal', 'external', 'navigation' etc. Another example is 'Safety Equipment' in Field 1 that provides Field 2 with the choices of 'First Aid Equipment', 'Fire-fighting Equipment' and 'Fall Arrest Gear'.
The ability to create a hierarchical structure of interconnected Custom Fields would benefit many of our projects and no doubt would be of benefit to other organisations, too.