I'd like to write an equation containing variables with subscripts, but my intention is that the subscripts simply identify the variable rather than refer to an array, in other words r(sub)s refers not to element s of array r but simply to a variable named r(sub)s. Is that possible?
Yes, it's called the "literal subscript" as opposed to the vector or matrix subscript.
For someone new to Mathcad its very often a subject for confusion.
In prime, btw, the literal subscript is achieved by typing "Ctrl -" a bit awkward unfortunately. But the enhancement is that in contrary to MC15 you can switch back and forth between normal and subscripted characters - especially usefull for chemists, I guess.