API:
The total number of view-through conversions. These happen when a customer sees a Display network ad, then later completes a conversion on your site without interacting with (e.g. clicking on) another ad.
Interface:
“View-through conversions” occur when someone sees your ad, doesn’t interact with it, then later performs a conversion action. For example, someone sees a video ad but doesn’t watch long enough to have it count as a view or engagement, or someone sees a search ad but doesn’t click it. If that person later performs a conversion action (like making a purchase or filling out a form), that would be counted as a view-through conversion.