Currently, if I am looking at the top line of a report in ARM and click on an expense amount, I might get a very large number of pages of detail behind that number. It would be useful to be able to drill down by the levels of the Unit set. So I could decompose that expense account first into my regions (say a $10,000 balance into $4,500 in region 1 and $5,500 in region 2) then regions into districts (Region 2's $5,500 into $3,000 in district A and $2,500 in district B) and then districts into departments (District A's $2,500 into $800 in Admin and $1,700 in Sales).
I can use the Unit Sets to select a level but then I only see that level, there isn't the context of multiple levels.
This was done very well (and was very popular) in the FRx report writer.