They are just different ways of estimating the actual IBUs in a beer, which is an actual, measurable thing (but expensive to actually measure, so we estimate). Tinseth is the most popular, I think, and probably the most accurate for our beers. Garetz is pretty old- not used by many people.
I can't be arsed to keep up this list of what's in the fermenters, but hey, check out the cool brewery I own!
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