You can create new tags in a few ways:
While editing a Test Case via the sidebar
While creating/editing a Test Case via Create/Edit form:
While creating/editing a Test Run:
While creating a Defect manually:
In Workspace >> Tags (tags created using this method can be applied to any entities later during edit):
Upon applying a tag to any entity, you can also choose one of the existing tags to be applied from the suggestions in the dropdown.
Existing tags can be found in Workspace >> Tags, where you can also delete them and see how many entities every tag has been applied to:
Note: Once a tag has been created, it can be used across all entities and projects.