Western riders shouldn't ride with their leg Off the horse. HOwever, they may have less leg on in the sense that they often have long spurs, so it takes a very small movement to tickle the horse, instead of a whole calf on to get forward movement in many English horses.
I would have a hard time memorizing that list. Please don't give me a test later, I'd flunk.
I tend to think of the aids as connected to the four corners of the hrose, and combine all four of them (well, wait a minute, with the seat, it's more than four, oh never mind!) to move the horse in various directions. But, I am no champ at this.
What I really wanted to say was the aid that most people neglect is the mind. Your mind and the horse's mind. If you really THINK what you want him to do, you'd be amazed at how perceptive he will be. And you don't bother asking him to do things until you've got his mind on you, then you'll have a lot less gray area blah type of reactions, and a lot more " I know what you want me to do and I'll do it" from your equine partner.
The other day I was trying to get Zulu to back up a hill, as I often ask him to do. But, he was so focussed on going home that he was leaning on my hands and dragging his feet. I stopped and kind of tickled the reins and spoke to him, "Hey! Get your head in the game!" and asked again, (after his ear had flicked back at me) and it went much , much better.