The Affiliate Sale Tracking Codes feature enables the affiliates to use their own tracking codes for sales (or to have any code executed once they refer a sale). You can activate this feature in your merchant panel Start->Features->Affiliate Sale Tracking Codes and after this feature is activated, the affiliates can specify some action tracking codes in Promotion -> Campaigns -> Action 'Edit Tracking codes' (the pencil icon). These codes need to be valid javascript codes or a call-back URL.

Each tracking code has to be approved by the merchant
(Affiliates -> Affiliate tracking codes) before it is used. After a referred sale the approved code is executed in the thank you page.
It also disables the cron job concerning sale/lead tracking,so all of the sales/leads (and clicks concerned with the particular sales/leads) will be tracked online (not postponed) which may cause your site to slow down if you have a heavy-traffic site.
Example Scenario:
There is a merchant having numerous affiliates. And e.g. there are affiliates āGā and āYā, who would like to track the sales made through their link/banner placed on their site on their own (so that it is forwarded to kind of their own script), so right at the time when a sale referred by them is accomplished a code would be executed towards them in the form of a 'hidden pixel', 'JavaScript' or 'call-back url'.
Available variables in the Affiliate Sale Tracking Codes: