Here is a recent thread on this topic that may be able to answer some of your questions:
Setting the Delta:Tolerance field to 0.000mil:100.000mil means the longest net in the group could be 100mils longer than the target net and the shortest net in the group could be 100 mils shorter than the target. So across all the nets you could have a variance in length as much as 200mils before seeing a DRC. (+/-100mil)
Setting the Delta:Tolerance field to :100.000mil (blank Delta field) means that all the nets could have a variance in length as much as 100mils with no specific Target. This is sometimes harder to match the members and could cause you to overly lengthen the nets.
Hope this helps,