Within TFSSecurity's reference page for permissions, under Team project level, there is a list of permissions and their corresponding actions. Using TFSSecurity, I was able to set all of the permissions at the project-level for a new group I created, except for Create Tag Definition. You can almost expect the error, with it's action being in upper and lower-case letters (Create), while every other action is in all upper-case letters (PUBLISH_TEST_RESULTS, DELETE, DELETE_TEST_RESULTS).
I've tried using create, Create, and CREATE, none of these work. Does anybody know the corresponding action for this permission, since it is not documented online by Microsoft?