7 is a piece of ferrite if im correct, its used to isolate electricity so that it doesnt interfere with other things, also used to soak up some electricity coming out of something. we used to use them on satellite radio antennas when i worked at circuit city, according to sirius and xm they said that then other cars would not be able to hear what you were listening to on your satellite, basically shortening the distance the transmission of the unit was.