Alberto Ferrari dives into a DAX operator:
If you like to follow best practices, you can just read this paragraph out of the entire article. If you are using SUMMARIZE to calculate new columns, stop. Seriously, stop doing it. Right now. Open your existing DAX code, search for SUMMARIZE and if you find that you are using SUMMARIZE to compute new columns, add them instead by using ADDCOLUMNS.
At SQLBI we are so strong on this position that we deliberately omitted a part of the detailed description of the behavior of SUMMARIZE in our book. We understand how SUMMARIZE works but we do not want your code to return inaccurate results, just because you use a function without understanding when its result might be different from the result you expected.
Read on as Alberto explains why, as well as the details of SUMMARIZE and how easily you can find yourself in a mess with it.