It's not that simple as the roll pitch yaw. There is no way to tell the angle of an object with respect to the vertical up. Using accelerometer only works statically. If you accelerate, accelerometer no longer tells vertical up but off into the direction of acceleration so considering its reading to indicate vertical up is no longer accurate. In this case, the gyro axes can help to make the orientation more accurate. That is what I imagine.
There are only 3 axis.
How you get 9 axis is beyond me!
There are three axes on the user/planetary/etc frame of reference. We usually refer to these as X Y Z.There are three more on the vehicle's frame of reference, generally about the Cg of the vehicle - there's your yaw / pitch / roll.
And do you think accelerometers measure acceleration?