Subfields (with RT::Extension::ConditionalCustomFields?)

I’m hoping to have two custom fields. The first would be a drop-down list of general topic areas, and one of these can be chosen. The second would be a drop-down list of more specific topic areas, and would be a multi-select list. (Think ITIL if you like.)

Ideally the second list would be constrained by the selection from the first list. I thought I would be able to do this with RT::Extension::ConditionalCustomFields but I’m struggling to understand the detail of the documentation.

  1. Have I got the right extension for the purpose
  2. How do I set this up so that once the staff member has chosen the primary topic item the secondary is automatically and immediately populated with the relevant subset of values?

Thanks