[bdk3clash]

I guess it depends on the cardroom, but most places consider bets of 1/2 or more to be actual raises that reopen the action, and less than that calls that do not reopen the action.

So when player B raises from $6 to $9, it's an actual raise. Player C called the bet from Player A + the raise from Player B, and Player D should have been able to re-raise to $15.