I'm using ATG 9.1. how can I know which version of JDK must be used for different versions of ATG?
With JDK 6 I am able to work normally, whereas ACC is not working as it Requires a small version of JRE.
Why is that only ACC requires a small version of JRE and not other modules?
ATG/Oracle Commerce's required versions can be found on https://support.oracle.com -- you will need to log-in, and you may need access to be granted by an Oracle representative.
ATG 9.1 compatibility is covered in document "Oracle Art Technology Group (ATG) Commerce Supported Environments - Release 9.x (Doc ID 1342327.1)", which will link you to https://support.oracle.com/epmos/main/downloadattachmentprocessor?parent=DOCUMENT&sourceId=1342327.1&attachid=1342327.1:ATG9404&clickstream=yes
The 'compatibility matrix' shows the required JDK is "Sun 32-bit or 64-bit JDK 1.6.0_41" on all supported platforms (Windows, Solaris, RedHat) for JBoss and Weblogic application servers, but different for WebSphere.
The ACC can be run from within the app server, or in separate JVM. I don't know a reason why it would require a different JDK version.
Oracle usually suggest that this as a base version, so if a minor update to your application server requires a minor update to the JVM, you should probably follow your appserver's requirements.