Hi Mike,
Here's a link to the
Release Video for GridView 9.0E, which was from about 2 years ago. The first topic describes Summary Filters.
A typical usage of Summary Filters would be to sort or select by subtotal groupings, such as the top 5 Salespersons this year based on summarized Sales Analysis transactions, or all Salespersons with total YTD sales exceeding 50,000. It's more than just performing an alternate summary operation on a numeric column.
Perhaps check the 'Customers with Transactions' workspace. My sample has a couple of summary filters in the Customer List view, though I can't be certain if I have edited the view since it was installed.