A: the difference between a 'contact' and a 'user' is:

Contact: A contact is an individual whose email address is stored in your mailing list or database. Contacts are the recipients of your email campaigns, newsletters, or automated workflows. They represent your audience or customers you want to engage with.

User: A user is someone who has access to the MailerCloud platform account itself. Users are typically team members or admins who manage campaigns, contacts, and settings within MailerCloud.