Some C++ value types have special requirements for the memory where they are located, e.g. specific alignment needs, or memory pooling. Since AngelScript doesn't provide that much control over where and how value types are allocated, they must be registered as reference types. In this case you'd register the type as a scoped reference type.
A scoped reference type will have the life time controlled by the scope of the variable that instanciate it, i.e. as soon as the variable goes out of scope the instance is destroyed. This means that the type doesn't permit handles to be taken for the type.
A scoped reference type requires only that the release behaviour is registered. The addref behaviour is not permitted. If the factory behaviour is not registered the script will not be able to instanciate objects of this type, but it can still receive them as parameters from the application.
Since no handles can be taken for the object type, there is no need to keep track of the number of references held to the object. This means that the release behaviour should simply destroy and deallocate the object as soon as it's called.
Unfortunately any function that either takes or returns the type by value in C++ must be wrapped in order to permit AngelScript to manage the life time of the values.
Here's an example of a function that takes a value and returns another and the corresponding wrapper.
Observe how the function is registered to return the scoped value by handle even though the scoped types really don't support handles. This is done because AngelScript will call Release on the returned instance after it is done with the value it received.