Why do some words have "X" as a substitute?

Solution 1:

In those three examples, there are three different, albeit related, reasons:

• Xing = Crossing. The "X" replaces "Cross" because an X is a cross.

• Xmas = Christmas. The "X" replaces "Christ" because the cross is a symbol of Jesus and because X (really Chi) is an initial for "Christ" in Greek (Χριστός).

• Xfr = Transfer. The "X" replaces the prefix "trans-" as it implies a crossing of something.

They are all abbreviations. I would be surprised if the origin of "Xing" wasn't in street signs, where "crossing" would be a long word to print to be able to read at a distance. I've more often seen "Xfr" as "Xfer"; it is used in electronic communications as jargon. I don't know the origin of "Xmas", but some people would have you believe that it's an effort to remove Christ from the holiday. I would guess it is just another general abbreviation.

Solution 2:

X is sometime used as a replacement target in technical document. Example:

Conditional jumps instructions (Z if zero, A if above,...)
JX : where X can stand for one of the value above.

Which means that Jx is actually either JZ,JA,... But it's a very rare case, I admit.