The superadmin user has special administrative privileges, so it is important to choose a secure alphanumeric PIN that cannot be easily guessed. For example, "ch33zeW1z" is a better PIN than "dog" or "123".
PINs for non-admin user accounts are typically numeric, since only numbers can be dialed. However, the superadmin account is generally used for administering sipXconfig via the web, where this is not an issue.