a, c, d, e, f, g, j, k, l, m, o, p, q, r, s, t, u, z.
H & W can probably be used pretty much freely, but they stand for Height and width, so i'm not that sure :/
then of course you could use...
aa, ab, ac... any combination you like, except for cos, sin and the others 😉
You can call them whatever you want eg. xmove=1; ymove2; yspeed=10 ect. The only time I've used letters was when one of my boxes was too long to save, so I had to cut down on the length.