In words like Tarxaanaa, chaTxanii, chaTax, and many more, it seems like the native (Indic) k/kh sounds were replaced with a x (خ) sound. Also, in may more words, k/kh was clearly left unchanged. I wonder why this replacement was done and what was any documented justification for it. Perhaps there are a few specific situations that call for it. If so what are they?