A coupla points:
You can install the Magnefine on the transmission out line or the return line. The main criteria for the choice would be the mileage on the trans. Even with a good trans, a certain amount of gunk will build up in the cooler(s). If the trans has more than about 30-50K on it, for that reason, you want it to install the Magafine on the return line to catch anything that might break loose from inside the cooler. On a new, or newish, vehicle installing the Magnefine on the out line of the trans prevents stuff from getting to the cooler in the first place and starting the buildup. On a reman trans, you ALWAYS install the Magefine on the return line because the failed trans may have left serious deposits in the cooler lines or coolers. Even if the cooler has been flushed or replaced.. on the return!