Marco Russo and Alberto Ferrari explain how the ALLSELECTED function works:
There are two ways to describe what ALLSELECTED performs and what its use cases are: a simple way and a complex way. In this article, we focus on a simple description of the function and its main use cases. We also provide details about when and how you can use ALLSELECTED without having to worry about the intricate details. If and when you want to go to the next level, invest the proper amount of time required to study this article: The definitive guide to ALLSELECTED. Be careful, the details are really complex and if you stick to the best practices, you can safely ignore said details. A real DAX guru should know all the details, but in this article we focus more on an introduction to ALLSELECTED.
Because of its inherent complexity, we describe ALLSELECTED by first focusing on a business case where ALLSELECTED is a good fit. We then provide an imperfect – yet useful – description of what ALLSELECTED is performing. And finally we provide the few best practices to follow when using ALLSELECTED. We will not provide the complete description of ALLSELECTED in terms of shadow filter context, not even as an introduction.
Even the simple solution is fairly complex.