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proposals/3060-room-labels.md

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+# MSC3060: Room labels
+
+Currently, the only allowed way in Matrix to advertise information about a room
+is through its name, avatar and topic, which can feel a bit limiting (a room's
+name is usually expected to be very short and isn't always very descriptive, and
+a room's topic is usually where one would find related social media accounts or
+the room's rules/code of conduct).
+
+On top of that, room names and topics are expected to be human-readable, which
+makes it difficult to use them for automation. One example of necessary
+automation that client developers might want to implement would be to filter out
+rooms allowing or dedicated to NSFW (Not Safe For Work, i.e. erotic or
+pornographic) content from the public room directory. Apple actually requires
+any application to be able to filter out such content in order to be accepted on
+their App Store.
+
+## Proposal
+
+### `m.room.labels` state event
+
+A new state event is defined, with the type `m.room.labels`. Its content
+includes an array of freeform strings used to describe the room and its
+discussions:
+
+```json
+{
+  "type": "m.room.labels",
+  "content": {
+    "labels": [
+      "archlinux",
+      "linux",
+      "support",
+    ]
+  },
+  [...]
+}
+```
+
+A label is expected to be fairly short, and to designate only one concept,
+therefore each label is limited in size to 35 characters and can contain only
+one word (a word being defined as a string containing any character except for a
+space (` `)).
+
+As a sidenote for context, the length of 35 characters is based on the allowed
+length for a topic on a GitHub repository (since the semantics of GitHub topics
+are very similar to the ones described here).
+
+Room labels appear in responses to `/publicRooms` requests, alongside other
+metadata for the room (member count, name, address, etc.).
+
+### `m.nsfw` room label
+
+A label is also defined, `m.nsfw`. Room admins must use this label to express
+that they're open to NSFW content being shared in the room. Clients can use this
+label to filter rooms allowing NSFW content out of the room list if needed or
+desired.
+
+Clients must display all room labels as defined, except for `m.nsfw`, which can
+also be displayed as the string `nsfw` or any case variant.
+
+## Alternatives
+
+An alternative to advertise topics could be through a modified
+[MSC1840](https://github.com/matrix-org/matrix-doc/pull/1840) (to allow multiple
+types for a given room), however the semantics for room types (how the room
+should be used by clients) are fairly different to those of labels (how the room
+should be used by humans).
+
+## Unstable prefix
+
+Until the feature described in this proposal is merged into the Matrix
+specification, implementations must use the unstable prefix
+`org.matrix.msc3060.labels` instead of `m.room.labels`.