Flagging is an in-built moderation measure in Discourse, intended to dissuade bad behavior and spam, as well as to promote civilised discourse. If you want to raise an issue about a post without derailing the topic at hand, the best course of action is usually to flag it.

By default, clicking the flag button will present a user with the following options:

  • Message @sam
    I want to talk to this person directly and privately about their post.
  • It’s Off-Topic
    This post is not relevant to the current discussion as defined by the title and first post, and should probably be moved elsewhere.

  • It’s Inappropriate
    This post contains content that a reasonable person would consider offensive, abusive, or a violation of our community guidelines.

  • It’s Spam
    This post is an advertisement. It is not useful or relevant to the current topic, but promotional in nature.

  • Something Else
    This post requires moderator attention for another reason not listed above.

Marking a post as Off-Topic, Inappropriate or Spam will trigger a moderator notification so moderators may review the issue in their flag queue (doc pending). Furthermore, this also “casts a flag” on the post, which has other consequences:

  • A user whose posts have accumulated 5 “agreed” flags can not reach TL 3. (with unique posts and unique users for each flag, confirmed by a moderator)
  • A post with 3 flags will be automatically hidden.
  • A new (TL0) user whose post is flagged as spam 3 times from 3 different users will have all their posts hidden as a result. (TODO: Should probably include inappropriate too)
  • If after 48 hours a flag is still in the flag queue without any action taken, an automatic mail will go out to contact_email.
  • A topic with 12 unresolved flags from at least 5 different users will be automatically closed until the moderators can intervene.

Source: https://meta.discourse.org/t/what-are-flags-and-how-do-they-work/32783