WHULorigan
Emperor Mongoose
I like the limitanei analogy
So the Sector Battle Fleet(s) are more equivalent to the mobile field army
A sector fleet is a waste of time, where is it deployed, how do you send it orders, how does it respond to events two subsectors away (months of comm lag). The admiral in command of the "sector fleet" may be able to command every other fleet they encounter, but they still don't know what the enemy is doing in real time.
No one command your fleet but the fleet admiral with the fleet, a higher ranking admiral could be several months away, and any orders they issue are based on a situation several months ago and won't be received for several months to come.
Sector admirals do not have god mode or author insight.
I think a lot of people do not fully appreciate the time lag with respect to fighting a war within a subsector, let alone across a sector.
It seems to me that the Sector Admiral is a mostly political appointment responsible for overall deployment and readiness of the various Fleets and Fleet elements within the Sector (Subsector and otherwise) at all times. The "Named Sector Fleet" is the name of the Regional Command (with Numbered Subsector Fleets and other Fleet elements assigned to it as needed, but I think it certainly could include specific oversight over and responsibility for additional core elements (Capital Squadrons and/or Task Units/Groups) that are scattered about the Sector in strategic locations that are used to form the cores of Task Forces and/or Battle Fleets during a major engagement. But it would be the responsibility of the Sector Admiral to appoint a Vice-Admiral from existing members of the Admiralty Staff to command such a formation locally and grant him broad powers to act as necessary as a mobile field commander in such times.