[Dudd]

I assume that it's just the standard aggression stat broken down by street, meaning that it's the ratio of (bet+raise)/call for the river only. I doubt that it can be used very easily to determine the frequency of bluffing on the end, as someone with a high aggression number could value bet a ton and rarely call, or they could call a decent amount but make up for it with lots of bluffs. Alternatively, someone with a low aggression number probably only bets their monsters and calls with everything else, but they could call with almost all made hands, yet bluff with the occasional missed draw.