I've noticed a lot of theories regarding why there was a space in "LA X," and I think the most simple explanation is that "LA X" denotes that we are in Los Angeles in a different timeline ("Timeline X" or "Reality X" or "Dimension X," whatever you want to call it).
"X" just denotes that we are talking about the "flash-sideways"/alt-reality characters.
For example: Jack is in the Temple wit the others, but Jack X is waiting to find out where his father's coffin is.
See? It's easy!
Sayid just came back from the dead, but Sayid X is on his way to (hopefully) find Nadia.
By adding "X" to the end of people's names, we can easily denote who we are talking about in which timeline.
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