bm only inspects the ‘top level’ definition of a command, for more complicated
expressions, and anything inside a { } group, bm forces bold fonts by essentially
the same (slow) technique used by the AMS boldsymbol command (but bm still
takes more care of the spacing).
@u13359 谢谢大佬,所以其实 效果上没啥区别是吗
@u28978 一个可能的区别可见这个链接... https://ask.latexstudio.net/ask/question/17446.html
感觉
\bm
更稳健一点点...