Contacts in your admin console. There are three recipient statuses: Subscribed, Pending, Unsubscribed. They will only receive messages from you if their status is that of a "Subscribed".
Every recipient in the system has its information saved into up to 40 custom fields. By default only 10 recipient fields are available in a MailUp console. The number grows to 40 with the Marketing+ packageCustom fields that can be added to a MailUp account. They are shared across Lists. They are currently listed to 39 (plus one used by default for email).
Status of a recipient that has "opted in" to a List.