Internal ID - Standard Field - user

any news?

Each object has both and External ID and Internal ID. The External ID is set by the system and cannot be changed. The Internal ID field is just an open text field that you can use for you own internal codification. There is no pre-build functionality based on the Internal ID that I'm aware of.

You also have a field called Alias which is used to set the email address for an object when using Interact emails.