Click Stats (for Affiliates) (affiliate mode >> reports >> click stats) is useful when you want to track the number of visitors, the number of sales, and the sales conversion rate. Tracking is important so that you can pinpoint the sources of traffic that are converting well for you.
If you click on a Funnel, you will see a drop-down list displaying Sub-ID and its corresponding stats. Sub-ID is for more detailed tracking. For example, suppose your affiliate link for the Sales Page is:
You can add a Sub-ID on the fly at the end of the url, like:
The Sub-IDs for the example above are: