ResolveAll
Historically Unity did not natively support resolving enumerable. It supported resolving arrays of dependencies but only if these where registered with name. Consider this example:
RegisterType(typeof(IService), typeof(Service));
RegisterType(typeof(IService), typeof(Service), "Name-1");
RegisterType(typeof(IService), typeof(Service1), "Name-2");
RegisterType(typeof(IService), typeof(Service1), "Name-3");
RegisterType(typeof(IService), typeof(Service1), "Name-4");
When calling ResolveAll<IService> (Resolve<IService[]>) Unity would only return 4 items. These with 'Name-x'.
IEnumerable
Above behavior does not work in all the cases so support for IEnumerable<> has been added. As opposed to ResolveAll resolving Resolve<IEnumerable<SomeType>>() will return all registrations satisfying IServece.
So in example above it will return all 5 objects.