I'll post a couple of pictures to show the definition of an absolute block tomorrow as this machine is too slow to upload the images. Basically the block is protected by a section signal and you leave it by a Home Signal, the distant just gives advance warning of the aspect of the the Section or Home Signal, allowing trains to travel at higher speeds.
I'll cover the communication first,
Each section has a block instrument with an indicator and a block switch, the indicator just repeats how the switch is set in the next signalbox that controls acceptance into the block.
The indicator and block switch have three positions Line Clear, Normal and Train On Line.
Normal is the usual position and is in the centre and the needle swings to either side for the other two positions.
Each section also has a block bell and a tapper, (which looks like a morse key), these are used to communicate what the train is between signalboxes. When you strike the tapper the bell rings in the next box.
Passage of a typical train, Between Signalbox A and Signalbox B.
A- Strikes 1 bell (Call Attention bellcode)
B- Repeats back 1 bell (Call Attention bellcode)repeated to confirm understanding.
A- Strikes 4 bells (Express Passenger bellcode)
B-Repeats 4 bells and places the switch on his block instrument from NORMAL to LINE CLEAR.
A- Clears signal into section and as train passes the Section signal, strikes 2 bells( Train entering section)
B- repeats 2 bells and places block switch to TRAIN ON LINE.
B- as the train passes clear of his Home signal and exits the block sends 2 pause 1 bells ( Train Out of Section) and places block switch back to Normal.
A-Repeats 2pause1 bells to confirm Train out of section.
There are lots of bellcodes and they differed between companies but the basic train classifications today are,
Class 1 train, Express passenger - 4 bells
Class 2 train, Stopping Passenger - 3 pause 1
Class 3 train, freight capable of 75mph or over - 1pause3pause1
class 4 train, freight up to 75mph -3pause1pause1
class 5 train, empty coaching stock - 2pause2pause1
class 6 train, freight up to 60mph - 5
class 7 train, freight up to 45mph - 4pause1
class 8 train freight or other train timed to run at 35mph - 3pause2
class 9 train, Eurostar Channel Tunnel Express1pause4
class 0 , Light loco or locomotives( no train) - 2pause3
These train classes are also used on the track circuit block system but are displayed on an electronic Train Describer so bell codes aren't used.
Bellcodes were also used to describe what the train is going to do, shunt into forward section, or for Obstruction danger etc. They were used before telephones were in common use and are still used today as they are faster and less prone to mis-understanding than telephone calls.