X refers to the sentence/context of the message you're replying to. (^) the direction of this symbol, will determend the direction of the message you're replying to. X^ means one message up
to the right (usually your own message). ^^^^^X means 5 messages up
to the left. This symbol can be displayed on it's own, usually this means that the person just wants you to
look at this message, but it depends on the context of course.
X can be used in capital or not, you often don't even use X on It's own, but it can be used for directions if you don't have a/any sentence/context.
X^ and ^X or ^^^ are mostly used in chats for practical reasons:
I'm fine
Are you fine?
x^
(This
conversation points to that you have all ready said that
you're fine and
you don't want to waste time on writing it again).