Email, Password (8+ characters) and Password Confirmation, plus acceptance of the Terms & Conditions checkbox, are always required. Beyond that, the business chooses which extra fields to show and which of those are mandatory from a list that can include Name, Last Name, Phone, Birthdate, SMS opt-in, Email opt-in, and Membership ID — so on some sites Last Name or Phone might be optional, and on others they're required with a red asterisk next to the label.