When does DAX Countrows() returns a 0 as a value instead of "blank"

Question

I was suprised to find the combination "..ISBLANK(Countrows(filter(...." in a measure, as I assumed controws() would return 0 if there are no rows, so this would always make isblank return True.

I looked it up in the MSDN function reference for Dax, and there it is stated that:

Whenever there are no rows to aggregate, the function returns a blank. 

This I had figured out myself, however I do not understand the next bit.

However, if there are rows, but none of them meet the specified criteria, the function returns 0.

What are "specified criteria"? I tried different combination of calculate and filter and countrows, but it always returns a blank value in the pivottable.

Carsten

P.S. Similar sentences appear in the Reference articles of other Count-functions, but I still do not see what it is supposed to mean.