SUM(IF Return > 0 THEN 1 ELSE 0 END)
COUNT(IF Return > 0 THEN 1 END)
In the first one we look through every row and if the value is >0 we return 1 if not 0 and then we sum the total value. Well if 503 were greater than 0 we have 503 1's which will sum to 503.
In the second one we are counting everytime something is greater than 0. It doesn't count when it is <=0 because we did not give it a value to use if it was <=0. When this happens Tableau treats the NULL values differently and ignores then in its count.
Either should work but both can come out slightly differently in our view depending how we are using it. As a rule, I would personally use the SUM function.