It's important to note that a Tag and an Audience are not the same thing.

• A Tag is a static “label” that can be added to, or removed from a contact.
• An Audience is a fluid group of contacts that share a particular attribute, or set of attributes. As attributes change, contacts can be dynamically added to or removed from the audience.

Note that Tags can be used to filter contacts to create an Audience but are not required.