SUMPRODUCT with COUNTIF to find unique entries

@ jason.b75

I did a simulation on the formula and found that if the CustomerID does not visit on the selected month. It will return as #N/A.

Below is the attachment of the simulation.

I could not figure out on how this below formula work. Appreciate if you could also share your understanding.

Formula:

=MATCH($A$2:$A$18&$F$3,$A$2:$A$18&$B$2:$B$18,0)=(ROW($A$2:$A$18)-ROW($A$2)+1)