The roles you assign users for working with documents and questionnaires allow these users access to the workflow statuses that correspond with their roles. For example, a document owner will see documents/questionnaires in the Draft status, a writer will see documents/questionnaires in the Collaboration status, and an assignee will see documents/questionnaires in the Published status. Which documents/questionnaires they can see in each of these statuses depends on a assigned security levels, as explained below.
All Users. When a document/questionnaire is assigned this security level, all users with the roles mentioned above can see that document/questionnaire in the workflow statuses they have access to, even if they are not specifically assigned to that document/questionnaire. For example, someone with the document owner role who is not a document's owner can see that document while it is in the Draft status, and someone with the reviewer role who is not assigned to a document can see that document while it is in the Review status.
Restricted - High. With this security level assigned to a document/questionnaire, only those specifically assigned to one or more statuses of the document/questionnaire have access to it. They can also access the document/questionnaire in any workflow status except archived. For example, someone assigned as a document's approver can see that document while it is in the Draft, Collaboration, Review, Approval, Pending, and Published statuses.
Restricted - Severe. As with the Restricted - High security level, only users specifically assigned to one or more statuses of the document/questionnaire have access to it. However, they can only view that document/questionnaire when it is in the status to which they are assigned. For example, an assigned writer can only see the document/questionnaire while it is in the Collaboration status.