Please can somebody clarify the real limits of an identifier? Because in one text passage there is a limit, in the other text passage not really.
You are not reading the first source correct.
It starts with :
The implementation shall be able to translate and execute at least one program that contains at least one instance of every one of the following limits:
And after the list there is a footnote:
- Implementations should avoid imposing fixed translation limits whenever possible.
That means, this section does not define any limit. It just demands the the compiler must support at least these numbers and there should not be any limit if possible. If you have sufficient memory to compile a program, then you should not fail to do so just because some identifier is rather long.
As a conclusion, this part
Currently I don't know which limits are useful.
can be answered by:
Your RAM is the limit. But make sure that you can handle at least the numbers given in the standard.