Typedef doesn't create a new type, it merely creates a new name for a type. I don't know what you mean by implementation defined integer type. If the implementation is conforming, then any integer type that isn't a basic C integer type, is really just a typedef for one of the basic C integer types ( char, short int, int, long, ... ).

In C, with CHAR_BIT defined as 8, unsigned char is the only type that satisfies the requirements of uint8_t.

( It is also possible that it is defined as char, if implementation defines char to have the same range, representation, and behavior as unsigned char. In this case, char effectively becomes unsigned char, but is still a distinct type. The types are compatible (can alias).)

Now why is unsigned char the only possibility. Type uint8_t is defined to have two'2 complement, have exactly 8 bits, no padding, and be unsigned. So we need an unsigned integer type. The unsigned type that follows unsigned char in rank is unsigned short char. But this type is defined to have ranges at least from 0 to 65535. Since CHAR_BIT is 8, unsigned short int cannot be used, because it has to have more than 8 bits. As there is no type in rank before unsigned char, it remains the only possibility.