ORACLE_SID is an environment variable which identify the System Identifier (SID) of the database.
The at sign (@) represents the ORACLE_SID environment variable. For example, to indicate a file that belongs to the current instance, run the following command:
SQL> ALTER TABLESPACE tablespace_name ADD DATAFILE [email protected]
You can create a syslog audit trail to track administrative activities.
The oratab file matches the oracle_sid with an oracle_home