It's part of the MediaItemStateChunk. My StateChunk-docs are not done yet, but you can find a preliminary one here:
https://github.com/Ultraschall/ultra...Doku.txt#L1104
BTW: I have recently added some functions into my API, who should make it easy to read my source-files I use for my Reaper-Docs, API and such.
As they include parameters, retvals, and tags and such, would you be interested into attempting a version of your great Reaper-API-docs, using my source-files instead of the original ReaScript-html-file?
My more detailed information combined with your search and filter and clipboard-features would make it awesome, I think.
I could help you to get your head around that, if you want.