Marco Russo and Alberto Ferrari cover the blank row:
We dedicated a previous article to the blank row in DAX. In that article, the goal was to explain the differences between VALUES and DISTINCT. This article here focuses on how important it is to generate the blank row to guarantee that totals are always correct.
The blank row is created for regular relationships that are invalid – that is, when there is at least one row on the many-side that does not have a matching row on the one-side of the regular relationship. The same does not happen for limited relationships, which do not generate a blank row in similar conditions. Therefore, if a model contains a limited invalid relationship, developers must pay extra attention to how they create reports to avoid obtaining inaccurate results.
Read on for an example of what they mean.