When connecting to an Active Directory, a service account is used so the Smoothwall can join the domain. The service user doesn't have to be an administrator user. The only permission this user needs, is the ability to add computers to the domain. A normal user account in a default Active Directory has the ability to add up to 10 computers to a domain, so a normal user account is often sufficient.
When setting up a cluster of Smoothwall systems, where the install count is close to or exceeds 10, this permission needs to be modified to allow more computers to be added. In addition, if the computer account is deleted and the Smoothwall is added again, this will count as a new computer, so if multiple rebinds have been done, the permission might need to be amended as well.