[usrusr]

Remember all those little two-letter abbreviations when 140 character SMS were dominating all remote communication, and those messages were still expansive? "Code" like that quickly establishes itself in narrow text communication and apparently the desire to make a quick buck by selling a "reference manual" isn't new either. My guess (from looking no further than the title I have to admit) is that this is a legitimate predecessor to stuff like this https://www.amazon.com/Text-Message-Abbreviations-Reference-... or https://www.amazon.com/-/de/dp/1467999466

In this case, the author(s) sure had a hand for wording!