1) It is a multiset generalization of Set 2) It is a supertype of Set and Seq (a Set can of course be build from a Seq). That is 'Set does Bag' and 'Seq does Bag'. Note that a Seq is a ready-made Bag and if it happens to have no duplicates it behaves like a Set. 3) It has set operations as generalizations of the Set operations
Note that this would mean that Seq would also have set operations.
4) It provides some Bag specific ops like (+) that return a Bag even when called with Sets
or Seqs. -- Jonathan "Dataweaver" Lang