XRAY - Model racing cars

I'm trying to figure out what certain things mean on this forum. I realized when the little graphic next to a topic is orange colored it means that there is a new message since last time I logged in or something like that. But what does it mean when some topics are in bold? Or when the little graphic is shaking? Is there an explanation somewhere that I could read?

take a look at the bottom of the XB8 forum section (the one with all the different threads listed)
there is a key which explains what all the different icons mean

Thanks but it still does not explain why some topics are in bold sometimes and sometimes they are not. I'm pretty sure this is a phpBB thing but I haven't figured it out yet.