Aim of creating user email document automatically on server

Hi everyone,

Today, I was trying out sync gateway admin api for creating users through on service call. I was able to achieve that but I also noticed a document created as well when I provide email in the request.

_sync:useremail:glenn@gg.com

{
“Username”: “abc”
}

Briefly, what is the usecase/aim of it? I tought it behaves like a look up id but I’m not sure about that. Could you inform me?

Thanks a lot.