Backfill Ranking Data is 3rd Party SERP data that is provided to use that matches a specific keyword, device, location combination to get historic SERPs. This allows you to go back in time as our ranking data is always moving forward. Most rank tracking solutions do not allow you to go back in time, so we wanted a way to get this information. Not all keyword, device, location combinations are possible, so we can not always go back in time, but for when we can, we will charge and provide the data as part of the rankings.