I do not know the original source of the code, but our congregation in Atlanta published a version a few years ago in their newsletter. The minister panders to the congregational prejudices, while planning on a long tenure. GP sermons are for more mature congregations. They are sometimes relevant to today's issues and they mayeven contain mild suggestions for change. R sermons are definitely restricted to those not upset by the truth.
Visitors are shocked.
X sermons have a limited audience. It is the kind of sermon that landed Jeremiah in the well, got Amos run out of town, and set up the crucifix ion of Jesus. Of course, such a code is more than humorous. The suggestion that a sermon is like a movie, points to a fundamental issue about worship. When I sit in a service, I find I am sometimes tempted to approach it in the same way the reviewers Gene Siskel or Roger Ebert approach a movie.