You may be thinking of the Compound File Binary Format[1], which is used by OLE applications.
OLE itself is an API mechanism, not a serialization format. So when an application that speaks OLE finds a chunk in a compound file that needs to be handled by another application, it would use OLE to go ask that application for help.
Compound formats, in and of themselves, aren't really all that special. One could make a case that this is a common use case for fairly prosaic technologies like XML and MIME, too, for example.
OLE itself is an API mechanism, not a serialization format. So when an application that speaks OLE finds a chunk in a compound file that needs to be handled by another application, it would use OLE to go ask that application for help.
Compound formats, in and of themselves, aren't really all that special. One could make a case that this is a common use case for fairly prosaic technologies like XML and MIME, too, for example.
[1]: https://docs.microsoft.com/en-us/openspecs/windows_protocols...