1. User's ability to add or read comments is primarily governed by user's level of permissions on the process.

• Can View or Can Edit permissions on a process (for example: Incident) allow adding and reading Public comments (or read Admin comments with User Level Comment setting checked, which makes it same as Public).
• Can Admin permission on a process (for example: Incident) allows adding and seeing Private and Admin comments.
  Note: Process History web part filter may need to be configured to show Private and Admin comments in this case (depending on process view page, for example this is the case for ServiceDesk Incidents and SD Incident View Simple process view page)

2. Private comment visibility and ability to add these is separately regulated by a Portal permission: ProcessData.CanViewPrivateComments