Both from a user perspective of getting notified of new posts in specific communities and of private DMs/replies/username mentions (keeping track of what you've already seen, and being able to filter out content based on keywords/topics that you aren't interested about)

And also from a mod perspective of being able to keep track of your community - new posts, reports, filtered content, modmail, etc. (ideally with a high enough default amount of items that arrive in the feed or a way to increase the amount like adding "?limit=100" on reddit)

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