The following capabilities are related to tags:
Note: The above capabilities may only be applied in the system context i.e. as global roles.
The capabilities tag:create and tag:edit should be set to allow everyone to create and edit tags. The reason these capabilities exist is because there were requests from some Moodle users to allow a greater degree of control over the vocabulary of tags (in a Moodle site for a high school it might make sense to block students from creating new tags).