在XML标准中定义扩展的最佳实践是什么？

I'm working on an XML Schema which is going to be used for data transfer between a number of applications, not all under our control. The core data is going to be the same for all, but we want to allow specific applications to store additional data to allow "round-tripping" of the files so they can save and reload and not lose anything that is specific to that app. What is the best practice for this?I'm working on an XML Schema which is going to