Nulls in rank

[#1]

while Using the rank function in a straight table.   I had taken 3 fields productID, Product Name, and in expression  count(Product ID) as Total Products  and rank column also in expressions.  but i was reloaded successful, and the result is same excatly what i expected ,  so one more column i added Month  the result is showing that some fields are null. the data and the application is attached below

[#2]

Re: Month sale wise rank

the expression is      Rank(TOTAL Column(1), 2, 1)

[#3]

can u please explain what is the 0 and 1 in the expression?