Purpose: what does this argument do, with relation to its target operator?
It may not always be obvious from the syntax example name of the argument as to its purpose when used. For a simple task like getting a square root of a number it should be obvious that the operator will need to know the number from which to obtain a square root. Sure enough, the sqrt() operator takes one argument 'number'. By comparison, the pow() operator for raising a number to a power;; it takes two number type arguments. Although the purpose of the two can be guessed, it is still necessary to know which argument is the number acted upon and which argument is the desired order to which it is to be raised.
Arguments are documented as part of articles describing discrete operators. Where possible, aTbRef will attempt to indicate an argument's purpose within a given operator, though the name used in short syntax examples may not give all detail about that argument. If unsure, the user may need to consult the full documentation article on the relevant operator.
At present there is no clear, unambiguous method for naming arguments that works across the whole set of operators, so if new to use of an operator, do read the note (and where necessary its linked notes) before first use.