Currently, the scope of tags is system wide. A tag created in one project is available for all other projects as well.
This is problematic when you run a Redmine instance with projects for different customers and those tags reveal customer specific names or systems etc.

It should be possible to set the scope for tags similar to versions, where you can specify if the version is usable for all projects, project tree, hierarchy etc.
At minimum, it should be configurable for a project, if the created tags are system wide or specific to the project.