C++: implementation-defined accepted physical source file characters

According to the C++14 standard,

§2.2.1.1 [...] The set of physical source file characters accepted is implementation-defined. [...] Any source file character not in the basic source character set is replaced by the universal-character-name that designates that character. [...]

Does it means that the C++ standard gives not implementation-defined or conditionally-supported support for non UCS/Unicode characters? For example, a physical source file encoding including characters without corresponding UCS code point.

The only think I can think of is, if that were the case (the compiler supports non UCS character through non UCS encodings), the compiler had to use the private UCS ranges to map those physical characters, but anyway, that solution doesn't fit to the "universal-character-name that designates that character" part, because universal-character-names inside private ranges doesn't define any specific character at all.