What is bit 6 of a character?

Here the documentation from Escape Sequences PHP.net (encoding the non-printing character (i.e SUB) to \cx for search pattern):

The precise effect of "\cx" is as follows: if "x" is a lower case letter, it is converted to upper case. Then bit 6 of the character (hex 40) is inverted. Thus "\cz" becomes hex 1A, but "\c{" becomes hex 3B, while "\c;" becomes hex 7B.

I'm kind of understand the conversion. I.e Z(1011010) the first 1 invert to 0 and give the result SUB(0011010). But isn't it should be bit 7 be inverted,not 6? And what hex 40(1000000) doing here? I know my consumption is wrong,i just really need your help. Thank you very much!