By rule: allow or block access to devices depending on the access rule and schedule. This menu item is available only for storage devices (hard drives, removable drives, floppy disk drives, CD/DVD drives).
Read only: allow read-only access to devices of the selected type. This menu item is available only for storage devices (hard drives, removable drives, floppy disk drives, CD/DVD drives).
You can configure access rules and schedules in the Configure device access rule window, which opens when you double-click the device type.