I've seen **** used when a run round is taking place.
The **** is what is technically known as an N.D.A (No Description Alarm) ...
A no description alarm is where a movement is taking place but no description has been entered into that actual berth etc, so the moment that particular track circuit berth gets shunted (& the track circuit becomes occupied) the signaller will receive an audible warning & a visual **** as an N.D.A. illuminated within that actual berth/section to alert them that a TC berth is showing occupied with no description interposed & to take action/update if required.
Sometimes as a signaller you would update it simply by going to the TD (Train Describer) keyboard & calling up that particular berth then typing.....say....0J00 (for example if it was a light engine going to Newton Heath) then pressing 'Interpose' to enter it onto the system....but in some cases like an unsignalled move or Single Line Working/bang road working it just wouldn't be worth it, as working in the wrong direction the berths wouldn't (in some cases) update themselves & travel in the wrong direction, so you would be forever manually updating the TD berths section by section as the train moved along, so for something like an unsignalled wrong direction move/SLW/loco runround etc you'd often just leave them all as NDAs then clear them all out manually after the movement has taken place.
Back in the 1990s when only we signallers could see them we sometimes (depending on who you were working with) sent our own unofficial messages in the berths
At Christmas it was usual to see MERY XMAS LINE SHUT
But if joking around sometimes we'd enter something like NEED BREW
Or if you couldn't get through to a neighbouring box/fringe box on the phone or something you'd transmit something to them like ANSR FONE
One one memorable occasion back in the 90s we,d had trouble with an ECS 142 that had failed in section between Deansgate Junction & Northenden Junction & after getting it assisted with another ECS (Going to the same destination, Piccadilly) I (after agreeing it with Northenden) changed the train description from something like 5H31 to something more fitting.....it was........5H1T
(Only Northenden Junction & myself could see this description), but for a few minutes & for around 3 miles of travel we had a 4 car ECS for Piccadilly running as "5H1T".
Other times when train failures/incidents have taken place I have seen the TD berths in other PSB boxes updated to........
NAKA (Knackared Failed Train)
JUNK (Same as above)
CRAP (Same as above)
*POS (Piece Of 5h1t....same as above)
*OOO (Out Of Order)
FITR (Fitters Attention)
NOGO (Not To Go/Going Nowhere/Do Not Use)
WATR (Lineside Flooding)
FIRE (Fire Brigade Attendance)
BASH (Bridge Bashed)