Contace setting for email, PRIVATE? What is this?

All of my contacts have 3 email addresses: EMAIL, EMAIL2, EMAIL3 and the ability to designate a primary. I noticed today that ONE of them also has a 4th email address labeled PRIVATE. What is this, where did it come from, and how did it get set?

Maybe it is imported from another application - did you import any of your contacts into eM Client?