Traders can be divided into groups. Each trade can belong to one or several groups. Division of traders into groups is necessary to divide traffic if the traffic on your site consists of different niches. For instance, if you have teen and mature traffic on your site(which is rather an extraordinary case), you can divide traders into 2 groups and point parameter group=mature

out.php?url=http://mature_gallery.com/&group=mature&p=50 

this means 50% clicks will go to

http://mature_gallery.com/, 

50% to traders from mature group.