I have come across this term several times. Often the resources jump right into explaining how to do something but do not mention what this is or what it is for. Also, a bunch of resources are in German or not very clear.
So,
- What is an eID?
- Is this a TYPO3 specific thing or is it a general concept?
- What can you do with it?
- Is this still relevant for TYPO3 >= 9?
Examples resources:
Either build a plugin you can put on your site, or an eID that does not need a site to function.
https://mcmap.net/q/2030345/-typo3-when-how-to-use-define-39-typo3_mode-39-39-fe-39
This resource actually explains it (though in German), but the code looks terribly outdated.
https://www.webmasterpro.de/coding/article/cms-typo3-eid-mechanismus.html
btw, there is a tag eID on Stack Overflow but it is for Electronic Identity Card :(