Thanks that's very helpful in answering my question and the extra info on the 3 aspects is quite fascinating.
I have other questions which betray my ignorance of signalling practice in general so perhaps someone can point me to a good textbook on the subject?
But essentially does the shunting signal control the same length of track (block?) as its respective semaphore? But I assume in some cases where the semaphore is red the shunting signal shows green to allow a loco to back off its train etc.
Thanks again, really appreciate the wealth of knowledge made freely available.