any number can be encoded as a hexadecimal string. which you can think of as a text string composed of the characters 0-9,A-F. there is a 1-1 relationship between the hexadecimal text string and bitcoin private key that it represents. Or just think of 256-bit text string consisting of 0s and 1s. That's a string too. or it can be thought of as such without causing any confusion or problem.
That's true but when you enter a private key WIF (the string) it still has to be converted to bits and treated as an integer but when you enter a mnemonic (the string) is treated as a string and even decoded using UTF8 when deriving BIP32 seed from it.Quote
Very good! Maybe I can locate them somehow. But you post so often that it might be kind of hard to dig them all up but it would be worth it I'm sure. 
It's usually in topics talking about mnemonic or when someone tries creating their own encryption algorithm. Here is an example from last month: https://bitcointalk.org/index.php?topic=5355541.msg57738764#msg57738764