I think there is a stickied thread at the top of every forum that explains how to label threads correctly. For example:
General Chat labels
Serious Discussion/ Advice labels
I don't know if/ where a master list is, but you can get an idea of the use just by looking at previous threads. :] Basically they're used to caution anyone who might look at the thread. If the label is "image heavy" in a general chat or picture board forum, it might be good for users on weak internet connections to avoid it. For other SI-related labels, they're so that people can stay safe when reading other threads and will only read them when they feel safe enough. Hope that helps.
