I just wish there were a little more explanation on this - how it is stored, when it is used, when do you have to do it again (e.g. when passport expires), etc.

When I did it, the scanner seemed to take it but the kiosk did not say anything (like your passport information has been stored and it will be used on future trips, etc.)

I also wonder if the photo is scanned and stored or just the Machine Readable characters?