It would be the same as any other situation where "value" is omitted and an extension is provided. (The extension might provide a translation, original text, a formula or a wide variety of other things.) The system decides if it wants to support the extension at all or ignore it. If it ignores it, then you can treat the element as if it had been omitted from the instance. If you want to store it, you might do so as a parallel column in your database or in a separate table with a "link" to where it applies in the primary table.