EmailDiscussions.com - Search flags

I see three predefined saved searches in my main page at the bottom of the folder list: mailing lists, notifications, personal. They are defined as follows:

Code:

mailing lists: flag:$ismailinglist in:inbox

notifications: flag:$isnotification in:inbox

personal:      NOT flag:$ismailinglist AND NOT flag:$isnotification in:inbox

I'm curious about the above flags and other possible values. Where is this info defined? I cannot find it. These flags and their syntax (e.g., leading "$") are not mentioned in the FM search documentation.