This was an OS-wide API. On the surface, it worked somewhat like Smart Objects in Photoshop, but rather than converting a file on embedding, you first "publishe

This was an OS-wide API. On the surface, it worked somewhat like Smart Objects in Photoshop, but rather than converting a file on embedding, you first "published" it and then the content could be embedded by any "subscriber" that supported/made use of the API. (Editing the file on the publisher side would consequently update any embedded content. Like in PS, any attempt to edit a subscription would open it in the publishing application.) However, I have no idea about any implementation details.