@Loy @stragu content warnings/content notices.
It's like a subject for a post/thread. Used to indicate the topic and things that might upset or harm others (like the description of violence) but also for jokes it just as short topic so the timeline doesn't get flooded by a very long post. Usually one needs to open a CW'ed postb with a button,n although it's possible to open all posts of a thread at once. Clients can be configured to open CW posts always.
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