All Facebook ad accounts are divided into three fundamental units: First, Campaigns, which are divided into Ad Sets, which are divided into Ads.Facebook tells us adsets:
An ad set is a group of ads that share the same daily or lifetime budget, schedule, bid type, bid info, and targeting data. Ad sets enable you to group ads according to your criteria, and you can retrieve the ad-related statistics that apply to a set.
So Ad Sets are part of Campaigns, which are organized by the same business objective. And Ad Sets contain Ads which have a title, description, link, and creative.
ReFUEL4 uses the ad set as the base unit for automation. This allows us to work with a small group of ads that that have the same bid type (we recommend oCPM) - Facebook optimizes the ad's display while ReFUEL4 keeps the creative fresh, so Facebook's optimization tools have good material to work with.
In our experience it is best to work with around 5 creatives in an ad set.