I typically see "dynamic power" being listed as the Max power rating, but that doesn't mean every company does that. Dynamic by definition is a variation of intensity, so they usually use the word "dynamic" in relation to Max power because with a large increase in "intensity" [of the signal] the amp is able to output that amount (but only for a very very brief period of time [transients]). RMS is the nominal amount the amp can output, kinda like an average of the power output at all intensities (large and small).
What specific amp are you talking about??