It was my understanding that the purpose of the classification of trains detailed in section 6 of TW1 was to enable priority to be accorded by signallers to higher priority trains (generally lower numbers, class 373 having been added at 9 after the system was set up), hence the lowest number being nominated post and parcel trains (I want to say it is the Irish Mail, probably because I've been tucked up inside nice and warm with Red For Danger), passenger expresses, snow ploughs, OHE and breakdown trains going to clear the line, and rescue trains. Followed by ordinary passenger services, then parcels and special freight, fast freight, ECS, and progressively slower freight.
Wikipedia reckons what I call special freight used to be fish trains, and newspaper trains used to rank with passenger expresses and trains going to clear the line.