Is there anything worse than rated R?

I'm curious, is there truly anything more intense or explicit than what's classified as rated R in the film industry? It seems like the boundaries of what's considered suitable for adult audiences have been pushed further and further in recent years, but I'm wondering if there's a limit to how far they can go. Are there any unspoken rules or guidelines that filmmakers and distributors must adhere to, or is it a free-for-all when it comes to pushing the envelope of what's acceptable? And if there is something worse than rated R, what would it be, and how would it be differentiated from the current rating system?