Why is the inertial block in simscape required for permanent magnet motors ?

Hi everyone,

I am fairly nnew to MATLAB and SIMULINK. I am trying to model Field Oriented Control of PMS motor. In MATLAB Tech Talk tutorials they say the inertia reference block is required. I have visited the documention for the block but I am still not sure what it does. Is it meant to simulate just inertial effects (which I guess are a loss) of a motor ? But the page also says that the torque is positive if inertia is accelerated in the positive direction, how do you know which direction is positive ? Is it anti-clockwise ? Thank you for your help in advance.