First off the R at the front of that stands for Resistor. The 12 is the heat range the Y is for the 13/16" wrench it takes I believe off hand and the C is for copper. Now the J and the C in front of the 12 on all 3 stands for the threads and length of the the tip. The only Mopar plug of them is the RJ12YC which is a Big Block plug with a decent heat range for a street or street/strip car.