[Q] About update.zip on sdcard.

I searched for this and I could not find an answer, so I ask it here.

On i9505 (Galaxy S4), the stock recovery says it can update from sdcard.
Obviously a signature is checked in the official stock recovery.
I guess the siging key is not public but held in a samsung safe.

What I wish to know is WHAT original file can be updated in this way?
Are there any original ones around?

Thanks for reading.
Zibri