For general questions about plays and theatrical performances as works of literature. If your question is about a specific play, use the title and author tags instead.
Literature SE has a policy to use medium tags on questions about specific works only if those works are too short to merit their own title/series tags. For example, a question about a short story would be tagged with short-stories instead of the title tag, but a questions about a play would be tagged with the title of the play and the name of the author. The theater tag is reserved for questions about plays and theater in general.