That is a peak power rating at 4 ohms mono. I wouldnt be too worried about this amp's output too much though; as I have still not seen one that ONLY does 600 watts. PG terribly under-rates thier products. If I recall, a few months ago either Car Sound or Auto Sound & Security did a review of this amp and similar amps and the 600ti actually tested at arround approximately 825 watts mono at 4 ohm. Dont quote me on the exact number yet, I will try and dig a little and see if I can find the exact review. Point is, at 4ohm mono this amp is a beast, at 2 ohm- even better, take it down to 1 ohm and wow.
As I said, a very fine amp...
Good Luck,
-zane