Autopilot: How does a visitor go from being anonymous to known?

All visitors to your tracked pages are initially treated as anonymous. They are all assigned a unique number so you can see who is a returning vs. a new visitor:

Visitors become "known" (sometimes referred to as being "tracked", "identified" or having a "session") when they:

When a visitor becomes known, all of their previously anonymous activity, as well as their future activity, will be associated with either a new or existing contact; this is decided based on whether a contact with the email provided already exists. For this reason, you must supply an email address in order for a contact to be created for a known visitor.

For example, let's say the anonymous visitor in the screenshot above submitted a form:

Notice that the previously anonymous visit is now associated with Peter.

Please note that tracking relies on cookies, so if a contact clears their cookies, they will be treated as anonymous until they become known again via one of the above methods.