The EI jumper is used when you have an internally resisted coil. I believe the internally resisted coils are 3 ohm and the coils that use a ballast resistor are 1.5 ohm. You don't want to run a 1.5 ohm coil very long without a ballast resistor. Sometimes there is a bypass for full voltage when the starter is engaged, but that would be it.
j.mccreight had another nice post on the subject here: