I think he is referring to the adapter plug that converts from the 30 amp twist style socket to the 30 amp RV plug. The companion model has the 30 amp twist socket on it. If he is using the Progressive EMS, he may also be referring to the ground/neutral plug that fools the EMS into thinking it is seeing a grounded neutral to keep it from tripping. This is a link that discusses this:
http://www.rvdoctor.com/2012/10/my-h...-power-my.html