The angles are interpreted as intrinsic rotations applied in the order Yaw, then Pitch, then Roll. I.e., an object would be rotated first by the specified yaw around its up axis (with positive angles interpreted as clockwise when viewed from above, along -Z), then pitched around its (new) right axis (with positive angles interpreted as 'nose up', i.e. clockwise when viewed along +Y), and then finally rolled around its (final) forward axis (with positive angles interpreted as clockwise rotations when viewed along +X).