Tier challenges are the long-form progression track. Each tier asks for more titles than the last, and a tier usually requires the one before it to be completed first.
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Tiers are about new watching and reading: they don't accept titles you've already completed, one title can't fill two requirements in the same tier, and most set a minimum total, such as 200 chapters for Manga Beginner's and 1,000 for Manga Intermediate.
Tip. Plan the long series early. A minimum total is easier to reach with a few long titles than many short ones, and a limited number of already-started titles are usually allowed.