How to add X-TAGS ? And what is the difference between X-tags and X-APIHEADER?

X-TAGS helps you to track and analyse group of emails which are tagged under one name while X-APIHEADER is a unique parameter passed to every email and helps to track the activity of each email transactions. Some of the examples of X-APIHEADER parameters can be Invoice ID, Transaction ID, CampContactID etc.
If you want to add X-tags then you need to follow this link.