I think training starts with CUES: specific word/action that is answered by a specific act. As training progresses the cues get combined: so now I'm using AIDS, if that makes sense. It CAN be confusing to a horse, if you've taught him a cue, like leg=go, and suddenly you want it to mean leg=yield. So you take it gradually, take the responsibility of explaining what you mean.
And I also believe the aids themselves change as you advance. By advance, I mean getting control of your horse's feet. I think most horses learn their leads better when your own leg "points" to the foreleg you want to lead with. However (and I've only taught this to one horse, so you can call me inexperienced!)-- when I started doing lead changes through the halt and canter, I wanted my horse to jump FIRST with her hind leg, to start the canter back there, so I began to prepare with the leg at the girth, but my horse learned to strike off with the leg behind. I'd gotten control of her hind legs.