You can start an Oracle instance in the following ways:
Using the default SPFILE spfileSID.ora
Using the default init.ora file initSID.ora
By specifying an initialization file init.ora
By specifying an SPFILE in the initialization file init.ora
When you run the startup command without a PFILE clause, Oracle reads the initialization parameters from the SPFILE. On AIX platforms, the default location for the SPFILE or PFILE is $ORACLE_HOME/dbs.