Classes may implement this interface to take advantage of automatic ODM (object document mapping) behavior in the driver. During serialization, the driver will inject a __pclass property containing the PHP class name into the data returned by MongoDB\BSON\Serializable::bsonSerialize(). During unserialization, the same __pclass property will then be used to infer the PHP class (independent of any type map configuration) to be constructed before MongoDB\BSON\Unserializable::bsonUnserialize() is invoked. See Persisting Data for additional information.
Even if MongoDB\BSON\Serializable::bsonSerialize() would return a sequential array, injection of the __pclass property will cause the object to be serialized as a BSON document.