At least in games I've played (pen&paper rpgs) 'open ended' is the usual dice (in this case, 1-6), with highest result (6) giving you a new throw (add to previous result) and lowest result (1) giving you also a new throw (subtract from previous result). Of course, it has a lot to do with commonness of dices labeled 1-6 and rareness of those with numbers from 0 to 5, so in a computer game it might be done differently.
Which makes me think, is the doms system 'open ended' (meaning, negative results possible also) or just 'high open ended' (only 6s (or 5s) result in an additional throw)? If I remember correctly, all the calculations are for the latter, so I'd guess it would be it.