Use of underscore _ in strings translations

Question

I believe that it is used to determine which keyboards shortcut will be used and in this example underscore defines that (even though I'm not 100% positive about that).

Let's say there is a string on weblate like this:

make all _uppercase

Does that mean that there will be e.g. ctrl+u keyboard shortcut for this command or some defined key + u?

If that's correct, I have following question, what should be done if for some reason there is no "u" letter in corresponding translated phrase?

Should it be translated without using underscore?

I've seen use of something like (_u) after translated sentence,

"corresponding phrase in other language (_u)"

Is it ok to do it like that?