A database is identified by a Global Database Name, and the instance associated with the database is identified by an Oracle System Identifier (SID).
Global database name - It is the name given to a database to uniquely identify it from the other databases in a network. The Global Database Name is composed of a database unique name (db_unique_name), a delimiter (a period), and a database domain name (db_domain). It is represented as: db_unique_name.db_domain.
where:
db_unique_name is the name of the database. It can contain a maximum of 30 characters, if the first eight characters are unique and begin with an alphabetic character. The characters can include alphanumeric, underscore (_), dollar sign ($), and pound sign (#).
db_domain is the computer environment used for the database. It can contain no more than 128 characters (alphanumeric, underscore (_), and pound sign (#)), inclusive of all periods.
Note: Ensure that the combination of database name (first eight unique characters of database unique name), delimiter, and the database domain name does not exceed 128 characters.
For example:
sales.us.example.com
where:
db_unique_name is sales
db_domain is us.example.com
Oracle System Identifier - It is a unique name for an Oracle database instance on a specific host. The Oracle System Identifier (SID) helps in identifying the control file, and locating the files required to open the database. When you enter the Global Database Name, Oracle Universal Installer automatically populates the Oracle System Identifier field with the database name. You can change this name in Advanced installation.
Oracle Univeral Installer limits the SID to 12 alphanumeric characters for single instance databases. For Oracle RAC databases, the SID prefix, which is the first eight characters of the SID, must be a unique name for each database. The SID prefix cannot contain an underscore (_), dollar sign ( $), or pound sign (#).
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Service Name (with Oracle RAC One Node) - Ensure that you provide a service name for the Oracle RAC One Node database. Service names allow clients to connect to a service, instead of connecting to a specific Oracle Database instance. Service names provide clients with continuous connectivity if an Oracle RAC One Node database is failed over, or relocated to another node.