Meaning of "reserved for the implementation"

Question

Reading the anwser from What are the rules about using an underscore in a c identifier I stumbled across the follwing quotation:

From the 2003 C++ Standard:

17.4.3.2.1 Global names [lib.global.names]

Certain sets of names and function signatures are always reserved to the implementation:

• Each name that contains a double underscore (_ _) or begins with an underscore followed by an uppercase letter (2.11) is reserved to the implementation for any use.
• Each name that begins with an underscore is reserved to the implementation for use as a name in the global namespace.¹⁶⁵

¹⁶⁵⁾ Such names are also reserved in namespace ::std (17.4.3.1).

What exactly is meant with reserved for the implementation?

Answer 1

Means exactly this. It means, that you are only allowed to create such names if you are providing a compiler or standard library implementation.