you have some symbol data this is what you want to send and that's an auxilary data this is random bits sequence the sender will do lossless compression the through the schema is fine receiver will get back out the symbol data and also get back out the auxilary data because you get them back out on this side you don't count it against your budget for encoding assumptions we make you we can compute P of Y given X which can be a strong assumption being able to find that distribution in your mixture model it's the distribution that you don't