char is always a byte , but it’s not always an octet. A byte is the smallest addressable unit of memory (in most definitions), an octet is 8-bit unit of memory.

That is, sizeof(char) is always 1 for all implementations, but CHAR_BIT macro in limits.h defines the size of a byte for a platform and it is not always 8 bit. There are platforms with 16-bit and 32-bit bytes, hence char will take up more bits, but it is still a byte. Since required range for char is at least -127 to 127 (or 0 to 255), it will be at least 8 bit on all platforms.

ISO/IEC 9899:TC3

6.5.3.4 The sizeof operator

1. …
2. The sizeof operator yields the size (in bytes) of its operand, which may be an expression or the parenthesized name of a type. […]
3. When applied to an operand that has type char, unsigned char, or signed char, (or a qualified version thereof) the result is 1. […]

Emphasis mine.