Let's understand the categories of customers in detail.
Who is a visitor?
Any unknown person who visits your website is a visitor, and you don't have any information about him or any open communication with him.
When does a visitor become a contact?
As soon as the visitor starts communication with you, and shares any contact information through which you can get back to him in the future. He becomes a contact.
Who is a verified contact?
When a contact signs up or logs in inside your app, he is identified as verified contact (customer). Any impersonation warnings which were being shown in his conversations will get removed after his verification.