Contacts
The four lists, what separates a contact from a lead, and how importing and exporting work.
The Four Lists
Contacts, Leads, Bounces and Unsubscribes — what lands in each and why they are kept apart.
Contacts vs Leads
A form submission is a lead, not a user — and leads are deliberately excluded from All Users.
Importing And Exporting
Bringing contacts in from a CSV and getting them back out again.