I imagine that either they have released that information, or that the messages come with some kind of descriptive metadata attached.
Entirely speculation though.
Neither. It's one hell of a lot of hard work. For the Hatch End - Milton Keynes signalling diagram, I had to use a diagram from the Watford South Junction Accident (1996) report, plus extensive local knowledge of Watford, TRUST reports (they show route number and line/direction), and a handy signalling guide to Kings Langley - Leighton Buzzard.
It took me about two weeks to lay out the berths on that diagram, and they might still be wrong, especially where a signal isn't at a station. TRUST has a timing offset for certain berth steps, so if the signal for, say, Cheddington is 0.5 miles further down the line than the station, when the train leaves Cheddington, it won't report - but when it passes the signal and a berth step message is generated, TRUST then derives the actual departure time from the station based on the train class.
I did similar for the core of the North London Line, being lucky enough to have a PowerPoint version of the Yellow Peril. For the Watford - Euston line, SimSig came in very handy, as did a Bakerloo Line route learning DVD. I can still hear the voices... "When signal Whiskey Sierra 3 4 clears with a shunt indicator, proceed toward the siding at a maximum of 10 miles per hour".
For other parts of the country - especially those areas where there isn't a SimSig data, if anyone can supply me with information about where the signals are, or route learning material, Yellow Perils, etc. - then it'll really help with drawing maps.
Most of it I can derive from TRUST data and TD berth step ordering, but, y'know - easier when you have the raw material.
Incidentally, TRUST will only show reports for trains passing timing points, TD will show every berth step whether or not it's a timing point or not.