Post notices are short pieces of informational text that are attached to posts in order to annotate some exceptional situation. They allow ♦ moderators or bounty donors to let other users know something about the post outside of the normal channels of edits, votes, comments and answers.
These annotations are not intended to replace the normal channels, but to annotate exceptional situations. Therefore, it's unlikely that a post will actually need an annotation. Comments, votes, edits, and new answers may be better choices. However, if you do think a post needs to be annotated by a moderator, feel free to raise a flag. (A word of caution: moderators will not be amused if you flag every answer you disagree with. Exercise restraint.)
With the exception of bounties and some locked posts, the annotation will remain until it is removed by a ♦ moderator. So if you think an annotation should be removed, the best bet is to fix any problems and raise a flag:
The notices are designed to be self-explanatory and provide relevant information. However, since you asked, I'll do my best to provide the "inside scoop":
There are essentially six types of post notices. Each has its own meaning:
This post contains content that needs citations from reputable sources.
Most common on Skeptics, this means the answer falls short of the site's criteria for citing sources. The best remedy is to edit the answer to included the requisite sources.
This post relates to a rapidly changing event.
The meaning of this is generally unclear outside of its original home (also Skeptics), but it has also been used on sites like Travel. Presumably the only thing to do about it is to ask in the comments.
Needs detailed answers
This question needs detailed answers, including citations and an explanation of why an answer is correct.
We appreciate the desire to help, but please consider either expanding your answer or deleting it. Most answers with this notification are really better suited to be comments in their current state. Most questions with this notice have already attracted multiple answers which failed to sufficiently explain themselves - try to avoid this pitfall if you post an answer of your own!
The bounty expires in [time]. Answers to this question are eligible for a +X reputation bounty.
This bounty has ended. Answers to this question are eligible for a +X reputation bounty. Bounty grace period ends in [time].
Congratulations! Someone else cares about this question enough to donate their hard-won reputation in order to get it answered. Enjoy!
There are a number of reasons for issuing a bounty, but they are functionally the same.
This post is a Community Wiki. Edit existing answers to improve this post. It is not currently accepting new answers.
Added to questions (especially old questions) that used to have lots of "sub answers" that are now merged into a single community wiki post.
Comments on this question have been disabled, but it is still accepting new answers and other interactions.
For whatever reason, people have used this post to basically chat about something other than the post. These locks are usually timed to expire after a certain period of time. They only block the addition of new comments; editing, voting, and answering (for questions) are still possible.
For more information, see the section "Why can't I comment on specific posts?" in How do comments work?
There are disputes about this (question’s or answer’s) content being resolved at this time. It is not currently accepting new answers or interactions.
Please see: What is a “locked” post?
This question and its answers are locked because the question is off-topic but has historical significance. It is not currently accepting new answers or interactions.
Please see: What is a historical lock, and what is it used for?
Other question states such as duplicate, migrated, closed, deleted, and protected also include boxes similar to those above, but are not listed among the annotated posts.