Asked via the review system (last time I will be responding to a question in there)
What do the icons represent?
As mentioned in the description of the plugin, they represent the options that have been chosen by the person in the list for their notifications. Inline, by default, is a bell (look at the top of this site....bell = notification list = inline) and email, by default, is an envelope (explains that, I think)
Where do these icons come from?
As per the settings, they are font-awesome icons. Refer to http://fontawesome.io/icons/ for the full list.
If someone has inline notifications for mentions enabled then, by default, a dark bell is displayed (fa-bell) and if disabled then, by default, a light bell is display (fa-bell-o)
If someone has email notifications for mentions enabled then, by default, a dark envelope is displayed (fa-envelope) and if disabled then, by default, a light envelope is displayed (fa-envelope-o)
How do I change them?
Use the plugin settings and replace the icon reference in the relevant fields. And don't leave off the fa- otherwise it won't display the icon....I may guard against that if people make the mistake too many times.