It’s probably documented somewhere, but you’d likely have to specifically know what you’re looking for since there’s no longer a book-style manual.
The # symbol prefix on constants is only for use in the Interface Designer. This lets you use localized constants in the interface to localize your app. When using constants in code you don’t use the # symbol prefix.
AppleMenuItem is from classic Mac OS and should have been removed years and years ago. Something something not breaking existing projects, but holy crap it’s okay to update your code once in 20 years.
It is probably buried in the user manual about localization
In many places in the IDE the # is required as a clue to the IDE that “this is the name of a constant NOT the literal string I want in here”
So you can use them just about anywhere a string literal could be used - not everywhere but most places
The examples are just ones that make it easy to have localized versions of menu items
Some are specifically set up to have something like “Options” instead of “Preferences” for user settings
Some are for things like “Quit” on macOS vs “Exit” on Windows
If you needed to cater to other languages then this is an easy way to do it since you can set a constant to be localized by OS AND language - SO in chinese on Windows you could have the proper chinse localization for “Exit” and one for Spanish on Windows and on macOS you’d have ot localized to say the correct term for Quit in Chinese or Spanish