The Data Level defines how detailed the rows in the report's main table are. With Campaigns (the default), each row is a campaign. With Ad Sets, each row is an ad set. With Ads, each row is an individual ad. Totals, summary cards, and charts still include everything, regardless of level. For client reports, the Campaigns level is usually the cleanest. Use Ads when the client wants to follow performance ad by ad. You can change the level at any time by editing the report, without changing the link.