Introducing ALLSELECTED in DAX

Introduction to ALLSELECTED, which is an extremely complex function to use in DAX.
Article and download: https://sql.bi/760153?aff=yt

How to learn DAX: https://www.sqlbi.com/guides/dax/?aff=yt
The definitive guide to DAX: https://www.sqlbi.com/books/the-definitive-guide-to-dax-2nd-edition/?aff=yt