pb:
If you’ll forgive me a little pedantry, the conditions indicated by the “red”, “green”, and “yellow” aspects, in raiload usage, are “stop”, “clear”, and “approach” (as in, “approach next signal, prepared to stop.”)
“Aspect” and “indication”, to be even more pedantic, have different meanings - the “aspect” is that which the signal displays, the “indication” is what it means. A semaphore actually shows different aspects in daytime (blade) and at night (lights) but they give the same indication.
Model Railroad Engineering, by David Marshall, from 1942, has lots of excellent information on signaling.
The lower-quadrant semaphore is, generally speaking, a two-indication signal, per blade. Straight out is Stop, drooping at an angle is Clear. The blade is weighted to return to the Stop position, and is pulled down to Clear. There were some lower-quadrants that could go vertical for Clear, but they were rare, except possibly as train-order signals.
A lower-quadrant semaphore signal can be arranged to give more than two indications, by putting multiple blades on one mast. Often there were two. The upper was the home signal. The lower, with a fishtail notch in its blade end, was the distant signal, which repeated the aspect of the home signal on the next block. This gave a three-indication signal:
Both blades horizontal - Stop
Top blade drooped, lower blade horizontal - Approach
Both blades drooped - Clear
The one thing I can’t remember is if it was possible to have the lower blade drooped and the upper blade horizontal - it would seem to be pretty meaningless.
Upper-quadrant semaphores can give three indications with one head.
Horizontal - Stop