"Mark all topics read"

This might be a stupid question but... what that "mark all topics read" option means?
I click it and nothing really happened.

Basically it should change the symbole in front of topics/threads title from
to


Even if you haven't opened and viewed the posts.
The symbole change happends otherwise as soon as you have opened and viewed a post.
Then when you have returned to the listview and hit refresh the symbole should also have changed.
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:0 Oohh! got it now. Thank you Svinto!