Reza Rad compares and contrasts:
These two functions in DAX are often used instead of each other; ALL and ALLSELECTED. Their behavior can be similar in some contexts, but it can also be different in other contexts. In this article and video, I’ll explain the difference between these two functions and when to use each in DAX for Power BI, Analysis Services, or Power Pivot.
Reza has a video as well as a blog post to describe the differences.