Represents an "unknown" type that will be known only at the moment of the serialization. Effectively it defers the choice of the serializer to a moment of the serialization, and can be used for contextual serialization.
To introspect descriptor of this kind, an instance of SerializersModule is required. See capturedKClass extension property for more details. However, if possible options are known statically (e.g. for sealed classes), they can be enumerated in child descriptors similarly to ENUM.