The 'X' character has been repurposed to represent the /sh/ sound.
In the current alphabet, the letter 'X' represents two or three speech sounds. /eh/, /k/ & /s/ while other speech sounds go without any character at all. 7M-English stops this Tom-foolery by reassigning the character. So now the /sh/ sound has it's own dedicated character. Think of "shushing" someone by puting an 'X' over their mouth (ssshhhhh, quiet please).