Base Directory: The directory where the Oracle Applications is installed. This is a generic directory.
I will suppose that the Base Directory is /APPS
$ORACLE_HOME (for the database tier): /APPS/db/tech_st/10.2.0
$ORACLE_HOME (for the apps tier): /APPS/apps/tect_st/10.1.2
$IAS_ORACLE_HOME (for the apps tier): /APPS/apps/tect_st/10.1.3