What is "icon3"?

In the 'subscribed threads with new posts' listing under "settings", sometimes there is an extra icon to the right of the newest-unread arrow: I have seen a 'thumbs-up' icon in the past, and today I see a lightbulb.

I haven't been able to work out what these mean, and the alt-text is just "icon3"!

Thanks!

I saw that stuff in the FAQ, but didn't realise that was what I was seeing, since the icons are only visible in the summary rather than the posts themselves (and it must be coincidence that I've only noticed them in "subscribed threads" rather than "what's new", which I tend to use in preference to direct browsing into the forum top pages where the icons are probably more relevant).