Topics allow your members to find other people and posts who are relevant to what they care about.
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Topics can be associated with posts, if a poster tags a post with them:

And topics can be associated with people, if they indicate they are interested in them on their profile:

When someone adds a topic to their profile (aka "follows" it), they will receive email notifications every time another person makes a post associated with that topic. They will also be discoverable on the topic's topic page.

Anywhere a topic is displayed on Switchboard, you can click on it to go to its topic page, which displays a list of posts about the topic and people following it. It also gives members the option to follow the topic:

Some topics (those which are official, which we'll get into in a minute) are also then displayed on the right in an A-Z sidebar that we call the topics index. This index is also accessible from the "Browse Topics ->" link to the right of the search and filters interface on the main post feed.

There are two types of topics: official topics and unofficial topics.
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​Official topics are suggested to members when they are adding topics to their posts and profiles. Only official topics are shown in the trending topics module on the sidebar of the main post feed.

On the A-Z topics index, only official topics are shown to normal members. To stewards, they appear on the A-Z topics index in a darker color.

On the left, we see how the topics index looks to a steward, who can see both official and unofficial (greyed out) topics. On the right, how it looks to a normal member of the Switchboard, to whom only official topics are displayed.

Unofficial topics are not suggested to members when they are adding them to their profiles or posts, shown in the trending topics sidebar, or displayed in the A-Z index. Only stewards can see unofficial topics in the A-Z index. (However, if an unofficial topic is on a post or profile, anyone can still click on it and view its topic page.)

Unofficial topics automatically become official if they are used 3 times on posts or 3 times on member profiles.
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Stewards can can also change a topic to official or unofficial status on its topic page.
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Changing an official topic to unofficial with the "Blacklist this topic" button will prevent it from being automatically made official again even after 3 uses.

Remember when I said there are two types of topics? That isn't entirely accurate.

There is a third type of topic, seeded topics, which function as a special subset of unofficial topics.

Seeded topics are a pool of 250 commonly used topics that we hand-selected based on their regular use across all Switchboards. Seeded topics are like unofficial topics in that they don’t display to normal members in the A-Z index. However, where unofficial topics automatically become official after 3 uses, seeded topics become official after only 1 use. (This because we have vetted their quality and decided they are useful or good in some way.) Seeded topics are also recommended to users who are adding topics to their profile or their posts even before the seeded topics become official.
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As a steward, seeded topics look like unofficial topics in the A-Z index: they are greyed out.

Blacklisted topics are topics you have blacklisted. They only display to stewards in the A-Z topics index, but their topics pages are still accessible if someone clicks through to them via a post or profile. Blacklisted topics appear as a special shade of red in the A-Z topics index.
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"Snakes. Why'd it have to be snakes?"