What does "rhel-x86_64-server-6-shadow" channel stand for and why i am not able to sync it on my Red Hat Satellite Server?

Issue

• What does "rhel-x86_64-server-6-shadow" channel stand for and why i am not able to sync it on my Red Hat Satellite Server?
• Satellite-sync form rhel-x86_64-server-6-shadow channel fails with this error message:

[root@satellite /]# satellite-sync -c rhel-x86_64-server-6-shadow
:
: 
16:57:03 ERROR: these channels either do not exist or are not available:
16:57:03        rhel-x86_64-server-6-shadow
16:57:03        (to see a list of channel labels: /usr/bin/satellite-sync --list-channels)