Why can CHAR_BIT usually be 8 in c++ Languages

Why can CHAR_BIT usually be 8 in c++

According to N4140(C++11 working draft):The fundamental storage unit in the C ++ memory model is the byte. A byte is at least large enough to contain any member of the basic execution character set and the eight-bit code units of the Unicode UTF-8 encoding form and is composed of a...
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