Easiest way to be sure is ask onyx support and with Arta posting here, maybe they'll respond.
My general impression based on behavior in other apps is that they might be tuned to page up/page down (codes
here). I've found them useful in scrolling through lists, like my apps in the android app store. But I wouldn't go on my general impression.
Also let us know if it does get added and which reader it is. FWIW, Moon+ responds to them. I haven't found another that does though.