External tags can be used to structure your templates as well as to store additional external data on them.
An example of how to do it via API can be found here.
Tags are available on Word, PowerPoint, Yaml as well as Version templates. There is no limit on the amount of tags per template, neither on its data.
Tags added through the UI create an empty data field for it, it is not currently possible to set external data on a tag with the use of the UI. However, it can be done by using the above mentioned API endpoints.
Adding tags using UI
Tags can be added by pressing the pink plus button. They can be removed by clicking the cross on the tag itself.