When you click on the dissociative disorders forum (for example) you will see the list of threads. If you are not signed in as trailrunner )ie - viewing as a guest) you will not see any check marks down the righ side of the table.
If you are signed in as you you will see a green check mark next to all of the threads you have posted in on that page. If you haven't posted in a thread there will be no check mark, signed in or not.
The check mark is a quick way to let you know what threads you have posted in. Every time you log out you will not see them. Every time you log back in you will see them again.