If I’m not mistaken, it’s not a countdown marker at all - it’s a ‘loop clear’ marker on an RETB line?
Do you mean a station limits board?
Its been a while since Ive seen one but I think the board is basically the same but the lines drop left to right.
Its blue so its not a countdown board either, or not one I remember.
With it having 3 bands I would say its for reminding the driver that the second countdown board is next.
In all seriousness though countdown boards can be used to count to anything, signals, crossings, switches even.
There's a run of signal countdown boards on the approach to Dewsbury in the Up direction. I'd always wondered what circumstances require them on the railway as they aren't found everywhere. Presumably they're used to assist drivers in places where the chances of a SPAD or missed-call is deemed to be more likely?They are yellow and black
Signal ahead countdown boards are white and red.