It has always been my understanding based on the usage in osrf/gazebo that a sensor of type force_torque may only be a //joint/sensor and not a //link/sensor. I believe it is not stated in the specification, and the test/sdf/sensors.sdf example even includes a //link/sensor of type force_torque. I noted this while reviewing the new sdf::ForceTorque DOM element, which does not enforce this condition, in keeping with the pattern currently set.

I would propose that we formalize this in SDFormat 1.8 and generate parsing errors if a //link/sensor has type force_torque.