Hi Errol,
It's a Zmax, version 2. Currently the Zmax numbers, like a lot of vv devices, are not accurate. That's why these devices seem to hit harder. You set it at, say 5v, but it is really giving you say, 6v.
RMS - root mean square - is a more accurate way of measuring the voltage. So if the RMS works, you should be able to read a more accurate voltage output. e.g. you set it at 5v but it is hitting harder than that. You look at the RMS menu and it tells you 6v. So that's what you are really getting - 6v, not 5v you set it at.
Since we haven't seen the version 2 Zmax yet we can't say how accurate the RMS will be. Also it is probably just a readout - not something you can program in. i.e. it will tell you 6v, but I don't think you can use it to accurately set the mod for 6v. Be nice if you can do that - but I think it will probably be just a readout function.