What is Future Extensibility?

The main objective of future extensibility is to design an infrastructure that allows the file format to be extended for representation of data structures in future versions of a given consumer/producer.

On the surface, that isn’t hard to do; there could be a special extension bit bucket allocated across every existing schema element, and any future extension could be placed there. However, the problem is more complex than that. The infrastructure must allow document interoperability between current consumers/producers and future consumers/producers, some which have not yet even been designed or built. That is, future extensibility involves building forward compatibility into the document infrastructure while remaining compatible with the current version.