The "AccountAdmin" role is one of the standard system roles. It will provide the User with the ability to update the settings for an account, including adding new Users and branches as well as updating the access controls for Users.
This role can be assigned to new Users by anyone with the account admin role, and is usually limited to a very small number of Users.
It is important to note that a User can have multiple roles at once. Thus it will often be the case that to provide a new User with the desired level of access controls they will need to be assigned a selection of roles.
The below table indicates the document types, record types and tabs a User can access when assigned with this role.
To see more information on how to restrict a User's access via roles, please refer to this article.