When writing formulae, how does one represent different "variables" of the same "type"? To explain further this question, suppose I have a formula with several references to times in it. 
For example, with a computer magnetic disk, there could be the time it takes for the head move from when end to the other; then there could be the time it takes the hard disk to make one revolution; the time it takes for i/o queuing and so on. 
Personally, I just use subscripts, like ts, td, tdr and so on, and this works fine for me but when it comes to sharing information with others, I'd like to use the correct conventions. Is my approach acceptable? 
 A: There are infinite versions of time to define. What about the time for half a revolution? Or a third? Or a tenth? We could go on. 
It is impossible to have unique conventions for each possible scenario. Instead we just use some often-used symbols for the "main" or "general" version and add subscripts like you showed. Or you can invent a completely new symbol for a specific case, if you prefer. 
Note that many variables do not have symbols according to any convention. Something as fundamental as a general length is given various symbols - often $l$ or $L$, sometimes $d$ for distance, in specific cases $r$ or $R$ for radius, $h$ and $w$ for height and width, maybe even a language-specific $s$ (short for "sted", meaning "place" in Danish) etc. Contexts, wording and even language will influence the choice. Of this reason you can never expect people to know what you mean purely from the symbol of a variable (except for in certain applied cases, such as labeled $\cos(\varphi)$ on generators e.g.), so you must always fully define a variable the first time it is used in an article, paper, or so. 
A: As long as you define them fully the first time you use them, that should be fine e.g. "The time for one revolution ($t_r$) and the time for i/o queuing ($t_q$) are..."
