Arnaud Le Hors has an interesting blog item On Open Extensions, from early February 2009.
It is related to Ken Krechmer's Adaptability Standards in that it mentions applications having an "explicit request mechanism", which I suppose means a downloadable handler, as a plug in to the application. Like downloadable CODECs for media players, I suppose.
It is also related to OOXML's Markup Compatibility and Extensions mechanism, in that it mentions mechanism that "the standard may prevent interaction altogether if the integrity of the information being exchanged depends on an extension which is not understood by the other component."
Le Hors makes the good point
Indeed, the key criterion regarding extensions is not whether some technology or product extends a given standard or not, but rather whether in doing so it breaks interoperability.
So what are Open Extensions? It seems they are like little mini-standards, where To be "open" extensions must meet the same criteria with regard to openness, transparency, availability, implementability, etc. I would guess, for example, that adding XBRL elements to an ODF document would be an example of an open extension, because the XBRL elements are defined publicly (well...that sort of thing): so Open Extensions fits into the XML namespaces system pretty well.
My MODUS Minimum Open Documents Using Standards concept is a little different here: in MODUS, there are no extensions allowed for data: however metadata can be freely extended, and alternatives non-standard formats for chunks of the data are allowed providing there is always a standard fallback version.
So MODUS addresses the issue "How do we make sure that all the data (not all the markup) can be interpreted as-is by a vanilla application" while Open Extensions is more "How do we make sure that all data (including markup) beyond what the main standard provides is in a form that is open for processing".
It seems a useful category.