Tags can be used to categorize contacts.
Difference between Tags and Segments
Tags needs to be manually set while segments are automatically assigned based on preset conditions.
Bulk Add/Remove Contacts from Tag
How to export contacts from tag
Tag in Workflows
Tags can be used triggers to start a workflow whenever a contact is added. There's also workflow action that allows you to add/remove contacts from tags.
Why we don't merge customer tags with ecommerce platforms?
Unlike product tags, customer tags in ecommerce platforms won't be merged into OnVoard as contact tags. This is to avoid conflicts since customer tags used in ecommerce platforms is used differently from contact tags used in marketing platform like OnVoard.
You can still view customer details like tags via contact associations. For example, see Shopify Customer Details in OnVoard.