As defined by Symantec a Active Scan, scan common virus locations, and etc.
The Ideia is that the active scan learn where the most common risk are in each company (for a give thresold - ex:1 month) so active scan will look for the past 1 month risk's location and get the path for the top 5 risks in the company and scan this path on the next active scan on all machines.
Since fullscan is not running all days, and for each company you have a diferent kind of "common" virus location, will be a great improve on pro-active virus detection.