Questions about Q_frame_class parameters

Hi,I try to lean VOTL ,So I buy a nimbus 1800 VOTL which is produced by Kris.I found the Q_frame_class=1(Quar) and Q_frame_type=1(X type),but the nimbus 1800 VOTL only have three motors with the front moter can tile .My understanding is that the Q_frame_class should be tricopter .

I don’t understand these two parameters, can anyone help explain the these parameters thaks!