On OS/2 and Windows 32-bit operating systems:
A default instance called DB2 is created when you install DB2. The instance name is used to set up the directory structure.
The following are set during installation: the environment variable DB2INSTANCE=DB2, and the DB2 registry value DB2INSTDEF=DB2. These settings establish DB2 as the default instance. You can change the instance that is used by default, but first you have to create an additional instance.
On UNIX:
There is no default instance on UNIX.