A Manager is an Investigator with the ability to create Managers, Investigators, or Viewers within your organization. They can view all Candidates in the system and view reports that reflect all organizational information. They can monitor the progress of all Investigations or just those assigned to them. If you have only one Investigator in your organization, he/she must be a Manager. If you want all Investigators to have the Manager role, that's fine and may be more conducive to an organization where multiple Investigators may work on multiple cases with minimum "ownership" of the case by the Investigator.
An Investigator is an Investigator with no ability to create other internal system users. They can only view and update Candidates assigned to them or shared by other Investigators. Investigators can add Candidates to the system and manage all aspects of the investigation including inviting references to the system to complete questionnaires. This is a good role to assign to an Investigator who simply conducts investigations assigned to them and manages their own caseload. If you have external contractors conducting background investigations, this is an excellent role for them.
A Viewers is an Investigators with view-only privileges. They are not really Investigators but you will probably assign this role to departmental managers who simply need access to the investigative records and reports. Viewers can access all Candidate records and view reports for all cases within the organization. There is no information in the system they can change, nor can they add any information. Since no Candidates can be assigned to them, their menu system has fewer options than other Investigators.
A Recruiter is an Investigator similar to a Viewer in that they have no ability to modify any data. The difference is the perspective they see when they log in. This role was created for specific customers with specific requirements. You may find it to be a useful role for your Viewers and it's easy enough to change a user's role to find out.