Threads with unread messages

If a thread holds messages that I haven't read yet, then the thread title is shown in bold in the thread list and there is an indication to "Go to the first unread post". There are quite some threads that I haven't even opened. So, these contain unread posts as well. I was wondering how this mechanism works? And how far does it go back? Is there a system setting or option to enable/disable this?
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