Adding an object to a SimGroup will remove it from it's current group.
Adding an object to a SimSet will not remove it from it's current group.
Said more specifically, a SimSet simply contains objects. When the set is deleted, nothing special happens.
However for a SimGroup, adding an object will automatically remove that object from any SimGroup it was previously in. In addition, deleting the set will delete all members.
Also, pretty much all objects have a
getGroup method which returns the SimGroup it is currently in, if any.