An unordered multiset works in a manner similar to a multiset, except that the internal behavior of these containers differs. A multiset makes use of red-black trees while an unordered multiset makes use of hash tables. The time complexity of multiset operations is O( log N) while that of unordered multiset operations is O(1). Hence, an unordered multiset tends to be faster than a multiset.
The values stored in an unordered multiset are not organized in any particular fashion, unlike in a multiset where values are stored in a sorted fashion. If performance is the criteria, unordered multisets are a good bet; however, if iterating the values in a sorted fashion is a requirement, then multiset is a good choice.