It dates back to Roman crucifix this war the normal cross figure actually in most areas this was done a little quicker or in sad regions so to speak. For public display cross where used in things but they looked more like a T in things then the modern day crosses.
The x also often had a circle around it for marking a spot on maps as to where you where or where the objective was or destination so to speak in things. This also is a play on Roman numerals in things as they understood that things changed 1 to 10 in things aka the start and end of something in ways a single line could be used or other in this likeness of things.