The Oracle System ID (SID) is used to uniquely identify a particular database on a system. For this reason, one cannot have more than one database with the same SID on a computer system.
When using RAC, all instances belonging to the same database must have unique SID's.
Switching between databases
Set the ORACLE_SID environment variable (or ORA_SID on VMS systems) to work on a particular database. Remember that the SID is case sensitive in Unix / Linux environments.