/forums/all at the very bottom of the page.
3.2 Icons Used on the Site
Some additional message icons that aren't yet included in the Icon Key: (You can hover over these also)
Represents news or info
Signifies importance that you are attaching to what you are writing
Denotes an idea
All of these options appear across the top of the composition window when you are posting a new topic or a response. You may select any one to include beside your post.
A red envelope means a thread you have never read. In some cases, a really old thread that has new replies may occasionally appear with a red envelope as well.
An orange envelope means a thread you have previously read which has new replies.
A white envelope means you have read the topic, and it has not been updated.
A ribbon is a followed topic.
A red bullet means a hot topic. The bullet appears next to the envelope:
Feedback received on this FAQ entry:
In other words, if you read a thread today, and come back tomorrow wanting to see only the recently added posts, clicking on the will do that for you. In the image below, the blue arrow is visible, and indicates that the member has previously viewed that topic, and new posts have been added since his last view.