Shutting down an instance (and closing the database that is mounted on the instance) takes three steps:

1. Close the database.
2. Dismount the database.
3. Shut down the instance of the Oracle server.

Oracle automatically performs all three steps when an instance is shut down.

An instance is started with startup.

When a database is started after a shutdown abort, Oracle peforms crash recovery.

If there is a logoff trigger, it won't be executed.

If there is a logoff trigger, it won't be executed.

Oracle records shutdowns of the instance in the alert.log file.