If you or a team members use email folders to organize your emails then SigParser can expose those folder names on the Contact records.
There is an Email Folders column on Contacts that exposes up to 15 folders that the Contact was discovered in.
For Google mailboxes the Labels on an email are uses instead of Folders.
If the folder name changes SigParser will see that as a new folder for any new emails discovered.
SigParser only filled the Email Folders in for Contacts with an email address in either the From or To fields. CC'd contacts are ignored for the purposes of applying Email Folders.