Here is how to seat reservations work on the Liverpool - Nottingham - Norwich route.
4 car units that work the Liverpool to Norwich services, reservations are only placed in the unit that is working the full service, the unit that detaches at Nottingham is always unreserved.
The unreserved carriages should always be the front two on leaving Liverpool.
If a unit is short formed then reservations will still be placed at Liverpool regardless, this is for passengers who have booked their tickets in advance and for passengers who might be reserved further along the line for example between Nottingham / Peterborough and Norwich.
On travelling from Norwich to Leeds on Monday we had 156406 on the 1357 departure and fair play to East Mids there was still reservations placed, maybe not what people had asked for due to a different layout. I was booked forwards table aisle and got backwards window airline, but who cares I got a seat.
Whenever I travel from Peterborough - Norwich or visa versa I always get allocated to carriage A, I presume with it being a two carriage service Nottingham - Norwich they try and keep most if not all reservations in one carriage leaving the other unreserved where possible.