Create a new Constraint Group or Constraint Class ?

When in Constraint Manager, Physical Domain, one can create a new Physical Constraint Class defining specific attributes for a custom rule set. One can then assing this new rule set to a set of nets. To do that it is instructed to create a new Net Class with menu Objects > Create > Net Class. Also on that same menu is available Net Group. Both options create a group that appear in the Constraint Manager Objects Name Column. I have triied both  options and cant really see the difference. 

The Question: What is the difference between creating a Net Class and a Net Group ?  What are the implications ?

Thanks for your help.