Sqaure

I was on another s2000 forums and someone said this:
There are a couple harnesses that go into the underhood fuse box. The largest is the main battery harness, which connects to both under hood fuse boxes and the battery. That one is a separate part # and costs over $100.00.

Does the underdash fuse box harness connect to the underhood fuse box aswell? Or are there 2 other harnesses that he is talking about that connect to the underhood fuse box?
What harnesses connect to the underdash fuse box and what are they called?
-Main battery harness??
-???

Thanks