Return                              Values

If                                  InStr returns
string1 is zero-length              0
string1 is Null                     Null
string2 is zero-length              start
string2 is Null                     Null
string2 is not found                0
string2 is found within string1     Position at which match is found
start > string2                     0

Argument       Description 

variant        A variable of data type Variant. 

valueifnull    Optional (unless used in a query). A Variant that supplies
               a value to be returned if the variant argument is Null.
               This argument enables you to return a value other than zero
               or a zero-length string.

               Note: If you use the Nz function in an expression in a query
               without using the valueifnull argument, the results will be
               a zero-length string in the fields that contain null values.