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.
Best Answer
You would write
\mathit{TP}or\mathit{FN}. If you're going to do this frequently, you can make macros for them.