It says "Hit the daily reputation cap on 50 days."
What exactly does it mean?
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To clarify the definition, it means you ended the day with 200 reputation or higher.
It's possible to hit the cap during the day and then lose rep later. If this happens you don't get credit towards epic/legendary for that day, even if you "left rep on the table" from upvotes received while you were at the cap.
This hits me particularly hard, as it's the difference for me between already having the legendary badge and still being some 40 days away.