Max(n1 [, n2 ...])
n2 (optional)
0
Evaluates the non-null occurrences of Field1 as well as the first occurrence of Field2, and returns the highest value.
The first expression evaluates the non-null occurrences of Field1 as well as the first occurrence of Field2, and returns the lowest value. The final result is the maximum of the returned value compared against the values of Field3 and Field4.

Max