In math mode, TeX assumes "hidden multiplication" (so that two variable names put nearby have invisible multiplication between), so that, AFAICT, expressions like `$ABC$` are rendered with small distances between letters.  

Now, what to do if I have a two-letter variable name, like for instance TP or FN (true positive or false negative)? Leave as they are? Put them in `\text`? Yet in the first option they will look like T\*P or F\*N and in the second one they will differentiate from one-letter symbols.

In optics, the 2-letter variable *NA* is used for numerical aperture.