While dealing with marketing database, you need to filter contacts based on multiple complex conditions and then group them under a common rule. Tagging rules help you handle such cases effectively. Tagging rules allows you to assign a particular tag or remove a particular tag when a contact fulfills certain criteria like Opened email, Clicked link, Browsed page, Filled Form, etc.
For instance, you can assign a tag "Active Customer" to all contacts who opened any email and filled any form. Such rules can be defined in Tagging rules.