Visitors are anonymous web browsers who have interacted with your online marketing content, such as your website, landing pages, or custom redirects. Visitors are converted to prospects when they fill out a form or form handler, which captures their email address and other information. Filling out a form or form handler is the only way to convert a visitor to a prospect. Running through completion actions, matching an automation rule, or visiting a Marketing Cloud Account Engagement landing page do not convert visitors to prospects, unless they also fill out a form or form handler on those pages. References: Visitors and Prospects, Forms and Form Handlers