Know X.
For a person you know you will not get an X back from.
Or if you're annoyed with someone,
or a guy you don't really know.
X
standard for any friend,
or a girl you don't really know.
XX a girl you like.
XXX plus flirting,
or playing a game with someone,
or going overboard.
Some people really do take notice of
the amount of X's they're getting.
If you send XX to a girl,
you may get XXX back.
But if you drop back down to an X after,
you almost certainly will not get XXX back again.
People get offended if you provide no X's,
unless you are renowned for being a no X.
Unfortunately,
in this practice you're dying as most
people send multiple shorter messages
rather than longer,
less frequent texts,
meaning that sign-offs are less necessary.