Some default subsystems are also classified as required subsystems. Required subsystems are critical to system operation; without them, IRIX will not operate. Inst prevents users from exiting an installation session if a required subsystem is available for installation but not installed (unless you choose to remove this safeguard).
When Inst is initialized, it makes preliminary selections of the subsystems to be installed during the session. If a target system contains no software, the preliminary selections always include all required and default subsystems; on targets with software installed, the default selections are all upgrades to the installed software that the distribution contains. When subsystems are displayed during an installation session, default subsystems are tagged with the letter d and required subsystems are tagged with the letter r.