答案1
因为代码有时比解释更容易,所以我将我的评论直接转换为答案。也许 OP 可以评论一下这种方法的问题是什么。
\documentclass[12pt]{article}
\begin{document}
Combine the subscript with the math term. Also, use upright for terms
that are not explicit variables. I might use $\phi_{\mathrm{SB},P}$
if P were a variable, or $\phi_\mathrm{SB,P}$ if it were not.
\end{document}