The software puts them there to indicate a "special post." Special posts are:

- Threads with a poll in them[list]
- Announcement threads (that are pinned to the top of a forum) [push pin]
- Threads that have been self-identified by the poster as possibly triggering [red X]
- Threads that have been moved [arrow]
- Threads that are very popular (by either views or replies) [flame]

I think that's all of them. All of these are done automatically by the forum software.

DocJohn