Verified answer

The n64 expansion pak is a red and black replacement for the jumper pak which is required for your system to start up, the little drawer in front of the cartridge slot houses it.

If you knew that already sorry but just to be clear.

There as only one expansion pak released its called just that you can get them cheap on ebay. It just allows the n64 to process better graphics at a faster framerate for latger games like majoras mask.