NQSconfig and Instanceconfig file:

NQSConfig.INI file is the first file you have to know when you are started working with Administration Tool.
This file contains the information that which rpd is currently running in online, cache information, authentication types and etc.,
After the installation you can see two folder in the installed path.
i.e 1. OracleBI  - Related Administration Tool and its configurations
    2. OracleBIData - Related to Presentation Server and its configurations
you can edit and can change the NQSconfiq.INI file whenever you want from the following path.
C:\Oracle BI\Server\Config\ NQS Config.INI

and same like, If you have created any new catalog in the presentation services and want to make that default, then you have change the one more file 
.i.e instanceconfig.xml file.
instanceconfig.xml file is available in the the following path:
C:\Oracle BIData\Config\instanceconfig.xml