What makes a 6v wiring system??

So, I’ve got a friend who had a new Ron Francis 6 volt WR-54 wiring harness he isn’t using and offered it up. So, my question to the brains on this forum who know electrical in and out. What makes make it a 6 volt? Wire thickness? What makes this specific to a car and can this be used to wire a basic race setup? I know barely the basics so looking for some information on this or links to things.

Thanks,
Jake
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