RACK is an incredibly common acronym in the BDSM world. If you are on a BDSM website, you shouldn't demand that people spell out every acronym already commonly accepted in the lifestyle. Again.. if you ask about it, I'm sure people will be likely to explain (shout out to Arria), but you shouldn't criticize people for using the acronyms without spelling them out. Not only that... but there is google, and wikipedia, and all sorts of other things that could tell you. I mean... the opening post used "SSC" without explaining it, so don't be angry if other posters behave similarly.