Hi Stewart,
"Only distinct values" does what you'd probably expect and only loop through the distinct values in the table/grid you are looping through.
For instance, if you're looping through your Estimate table and want to count the number of customers you've created estimates for, you can loop through the esimtate table, only showing distinct values for the field "Customer". By doing this, if you had a customer with several estimates, the loop would only loop through the first instance of the customer and skip any other records it might otherwise loop through. Here's a screenshot of how I set this up:
In my example, the other actions in the action set are basically appending the customer name to an action result, just simply to show that the customer name is only being shown once. This worked for me. I had several customers in my test acccount with multiple estimates and each customer name only showed once.
Hope this helps. Let me know if you have any other questions.
Adam