Tags are a very efficient way to categorize emails you are sending with Critsend. Unlike unique identifiers, they enable the creation of analytics and reports based on each of them. For instance, if you use CritSend to send invoices, password reminders, and newsletters, each type of delivery should be tagged. You can also use tags for segmentation purposes, such as tagging by country, gender, or any other customer feature relevant to you.
A tag itself is simply an ascii alpha–numerical string under 100 characters. The dash (-) character is allowed but no other signs are (including underscore, punctuation marks, and so on). If you do not use any tag for an email, it will automatically get tagged as "default". Examples of tags are: “invoice”, “forgot-password” or “june-newsletter”.
For more information on how to set up tags in Critsend, read this post.