'Q' current is torque producing current - in torque mode, 'D' is commanded to be zero and 'q' is commanded to be the necessary current to produce the desired amount of torque (calculated from Kt). The 'Q' vector points in the same direction as the direction of motion (tangential to the shaft).
'D' current is 90 degrees to 'Q' current ie faces outwards from the shaft. If we have all 'D' and no 'Q', it puts the current vector outwards and 'locks' the shaft. In open-loop mode (autorun), 'D' is commanded to be 90% of continuous current and 'Q' is commanded to be zero. Then the commanded angle is rotated and the shaft appears to rotate.
In an induction motor, the flux is generated by 'D' current. For a Brushless DC motor, this is generated by the magnets, so 'D' is set to zero when in closed-loop mode.