Letters Make Buses [More] Confusing

My knowledge of buses is entirely restricted to the 743, 744, 721 and the 722; beyond them, I am whatever is the bus version of illiterate [and even within them my knowledge is lacking]. The 743 &744 are simple and striaght forward [and conviently stop right in front of my house], but the 721 & 722 are less so.

Here is my basic understanding. Both travel between the city and noarlunga interchange, stopping at both the reynella interchange and flinders medical centre along the way; and between these two locations they're course is virtually identical. Before the reynella interchange, the 722 takes a very long, roundabout course, while the 721 is much more direct. The 721 is always packed. And that's it.

While this small [lets say, four inches] amount of knowledge is enough for my purposes, the whole situation is made much more complicated by the extension of the bus number with a letter. What is the difference between the 721 and the 721A?? I mean, I get that the 721F and 721X are meant to stand for "Fast" and "Express" respectively, but one day I came across the 721R. What the hell is R meant to stand for??

Retarded?? Randy?? Is it some sort of sex-bus?? [If there are any porn writers reading this, I want my name mentioned.] Why would there be a bus of sex?? What if there are old people on it?? Why have I completely changed topic??

Well I've gone off at a tangent, coz I actually know that it is supposed to stand for reynella. This means that it follows the same path from the city to the reynella interchange, but stops there. So it's effectively the 721 with the tip snipped off.

Kind of like a circumcision.