A normal user can log into iTWOcx and create, view and respond to correspondence, upload and search the document register and create saved searches.
An external user can receive items addressed to them via email but cannot log into the system. External users are good for project participants that you want have limited interaction with system but still want to keep informed.
The company administrator can add and retire users for their company and edit user and Company details.
Project administrators are in charge of running of iTWOcx site can make changes that affect the entire project. This includes changes to configuration of correspondence, the document register, additional modules, adding new companies and new users, applying security rules like who can talk to whom and using project utilities. By default Project Admins belong to all other admin groups such as Module and Company.
To find out more see Project Administrator Resources
These are special admins responsible for configuration of core and additional modules such as Document Register and Contracts Administration. Module admins are best used when you want to give a user Project Admin like rights but for a specific module instead of the whole project.