The third parameter, filterItemIndex, checks whether the index is 0 or not; it checks whether the "All" item is selected (which is always the first item in the list and so always has index of 0) or not. This uses an index (integer) as opposed to a boolean because it is exposed as a public method and the column may or may not be selected. If the filter is a single item (or none), then the string in the second parameter, filterString, is enough information. The filterString parameter should be an exact match for one of the strings returned by the GetDropDownFilterItems method for the column.

This example filters the second column.

Reference

SheetView Class
SheetView Members
HideRowFilter Class
StyleRowFilter Class

User-Task Documentation

Managing Filtering of Rows