Piggyback Pixels is an advanced feature of tracking integration with the brand(s) you are working with, similar to Event Postbacks. When a piggyback pixel loads on the order confirmation page, the tracking software you use will signal that a conversion has occurred. The pixel only loads when a matching cookie for a specific brand is found in the browser.
Piggyback pixels will load on the order confirmation page before impact.com can process the action. This will tell you that customers clicking on your link were part of the conversion process, but does not guarantee that you won the conversion. If a customer leaves the order confirmation page before the piggyback pixel fires, a false negative result occurs.
Note: Piggyback pixels served on a non-secure page will not load (so sites served with http). If you want to use piggyback pixels, you'll need to serve them on a secured page with https. You should consider using a postback URL as your conversion tracking pixel over piggyback pixels.
Postback URLs can be used on every campaign, and in every account, and are set up using a URL structure that matches your 3rd party tracking system specs. Piggyback Pixels can only be used on Advertiser campaigns that allow them and are set up using a javascript pixel that is instructed to fire directly after the Impact pixel (if present).
Another strong argument for using a Postback URL is its ability to handle and pass more data points than a Piggyback Pixel. The reason for this is - because a Postback is handled during Impact action-processing we are able to include referring click data points like SubIds, SharedId, AdId, etc. in your Postback URL while a Piggyback Pixel does not have these data points available because the pixel fires before Impact action processing.
The primary reasons you will not see the Piggyback Pixel option are either; the Advertiser you are working with does not use pixel tracking (therefore there is no pixel to piggyback on), or the Advertiser has disabled the piggyback option in your Contract. Some Advertisers will choose not to enable this option for reasons of slower page load times, internal restrictions on website pixel fires, the possibility of false negatives, etc. Please reach out to the Advertiser directly if you'd like to discuss enabling this option.