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This was confirmed on the Weeklymtg stream.

https://www.twitch.tv/videos/2265055461?t=00h35m03s

Gavin: "One thing that Brawl does on Arena is, it kind of buckets your deck into 4 different categories. We worked on (edh) brackets before I learned this btw... It seems to be working decently well for them. It pairs you against people that have a similar benchmark of cards. I think there is a lot we can learn from that."

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about 4 hours ago - /u/WotC_Jay - Direct link

Originally posted by Astigmatic_Oracle

I think it also cares about wait time, so the longer you wait the more likely you are to be paired out of your bracket to reduce wait times.

This is accurate. All matchmaking on Arena (or any game, really) is a compromise between finding a good pairing and a quick pairing. The longer it's taking to find a match, the wider it can go.

about 3 hours ago - /u/WotC_Jay - Direct link

Originally posted by GavinV

Hey all! Just to clarify what I was talking about today, the four bucket system isn't literally how Arena Brawl works - it's more nuanced and conceptual. As I even mentioned on stream, I just learned about this myself as we dug into this concept so I might have been a little off. Just want to avoid you all going down rabbit holes that aren't quite right. Hope this helps clarify and happy Brawling!

(Wotcstaff)

Mods, this is actual-Gavin-Gavin, and worthy of flair