The unibyte and multibyte text representations use different character codes. The valid character codes for unibyte representation range from 0 to 255--the values that can fit in one byte. The valid character codes for multibyte representation range from 0 to 524287, but not all values in that range are valid. In particular, the values 128 through 255 are not legitimate in multibyte text (though they can occur in "raw bytes"; see section Explicit Encoding and Decoding). Only the ASCII codes 0 through 127 are fully legitimate in both representations.
t
if charcode is valid for either one of the two
text representations.
(char-valid-p 65) => t (char-valid-p 256) => nil (char-valid-p 2248) => t
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