i'm not a plumber, but i seem to remember from years ago, aboard an oceanagraphic research vessel, (i was a ship's electrician, in another life), this difference was defined as "when the hot water heater becomes a "pressure vessel" with a test pressure certification as such, then its no longer a "heater", but is a "boiler". also, if memory serves me, it must be capable of maintaining a certain "saturation" temperature at a given "working" pressure.

hope this helps...my memory isnt as good as it was 30 years ago... [Linked Image]