Per the serialization spec:

• Root documents must be serialized as a BSON document.
• MongoDB\BSON\Persistable objects must be serialized as a BSON document.
• If bsonSerialize returns a packed array, serialize as a BSON array.
• If bsonSerialize returns a non-packed array or stdClass, serialize as a BSON document.
• If bsonSerialize did not return an array or stdClass, throw an MongoDB\Driver\Exception\UnexpectedValueException exception.