Headcodes explained.

0 - Light locomotive(s)
1 - Express Passenger train,
Nominated parcels tains,
Breakdown train going to/returning clear the line (1Z99),
Traction unit going to assist failed train (1Z99),
Snow plough going to clear the line (1Z99)
2 - Ordinary Passenger train
Breakdown train not going to clear the line (2Z99),
3 - Parcels train,
RHTT,
Freight train running at more than 75mph
Specially authorised ECS
4 - Freight to run at 75mph
5 - ECS
6 - Freight to run at 60mph
7 - Freight to run at 45mph
8 - Freight to run at 35mph or less
9 - Class 373 trains & specially permitted express passenger trains

S - Inter-regional train terminating in Scottish region
M - Inter-regional train terminating in Midland region
V - Inter-regional train terminating in Western region
E - Inter-regional train terminating in Eastern region
O - Inter-regional train terminating in Southern region
Z - Inter-regional special or short notice trains
P - Inter-regional train terminating in the Anglia region

North West internal headcodes:

N - Train terminating in the Preston, Blackpool, East Lancashire area
H - Train terminating in the Manchester area
F - Train terminating in the Liverpool area
C - Train terminating in Cumbria or North Lancashire
B - Blackpool Nth - York services
T - Test or training trips (also used for short notice services)
A - InterCity service operating wholly within the London Midland region (Preston-Euston)

The last two digits identify the particular trip ie

2F55 is an Ordinary Passenger train heading for the Liverpool area.

The last two digits may be reused 6hrs after the last train of the same number terminated