Under fleet management in addition to "default" you have the option of adding additional fleet categories of your choice.
I have experimented with categories -
aircraft type (prop, jets etc)
cargo
new leases
out of service/lease cancelled
recent changes
The major drawback is that the "operations control" only lets you show individual fleets .
I was wondering how other people use the fleet option.
Depending on the airline - in some airline I used to have one fleet category for each aircraft type. In other I had one fleet category for each hub. And additonal I had a category for aircraft I want to change something in the timetable…
As airlines grow, I usually try to cluster by hubs and destinations, e.g. "MAD domestic" "MAD Euro" "MAD Interconti".
I usually group them by aircraft types andjust label them.
Yeah, that’s what I do too when I have multiple hubs. With my current airline, I made the split by aircraft type and destination (although admittedly, splitting up between DH4 Europe and 737 Europe isn’t all that effective, since neither of those will operate on the intercontinental market).
I’m kinda hyper organized, but for me it just depends on the current size or expected size. Also I have even numbered flights going out-bound a hub and odd in-bound… further grouped into hub/destination/aircraft type. 
Yeah, flightnumbers are also organized that way with me. Before planning, I create the routes, so my list of available flights is quite long (probably about 150 flights in it right now), but then I can be sure that flights are planned properly.