KTX = ?

I’ve encountered KTX markers in some modules. A search of the rule books for the games in question hasn’t led to a definition. In fact, the paper games don’t contain such markers.

I searched the forums for “KTX markers” and got only one hit, which did not explain what the term means or what the markers are used for.

What does KTX mean? What are KTX markers used for? They come in variable colors, which is even more baffling.