Managers and Approvers have different levels of access to subordinates' information. The amount of information available can be customized from the Permissions tab in the Company Profile app.
Managers can have full access to view a subordinate's information, including sensitive information (compensation, equity, and I9 documents). This information can also be restricted from all managers or specific managers through the Permissions tab in the Company Profile app.
Approvers will only see information pertaining to the specific request through the generated request email. Approvers do not have any special privileges to access any other information. For example, if the the request is related to salary or stock options for a specific employee, the approver will only see that information in the request email.