I quite didn't catch yet how char is represented regarding its sign. What I got so far is that char, much like any other integral, can be sign or unsigned. But unlike other integrals the decision of which is the default representation is left to the compiler. So far so good.
However, I read repeatedly that there's a third type of char; a plain char. And that this type cannot be represented. However I wonder why it is mentioned and what does it actually mean?