[Question] What is meant by Simulcra Levels?

Simulcra levels are a concept that have seen a lot of play on Less Wrong. I suspect that different people are using these concepts in slightly different ways, so I thought it might make sense to ask a question to provide a central location for recording these theories and helping people disambiguate. For this reason, I don’t think that there should be a single answer, but rather multiple answers. If you propose a theory, I’d suggest ideally providing a descriptive title or label.