Resource for Story mission ratings? (Gold, Silver..)

OK, I searched, couldn’t find what I’m looking for.

When you look are your stats and see the Gold, Silver, Bronze what do they mean?
i.e.
Gold: You kicked every butt and didn’t die
Silver: You didn’t die but you did it too fast
Bronze: You did it but they kicked your butt.

It’s purely score based. The higher the score, the better the chances at silver or gold.

Thanks a bunch!