does anyone know what this is ??

[jiyoung Kim]

                   I don't know since when its there but I just found it out.

but I could not find any information of it on User guide and release notes.

[jrabinow]

This allows domain information to be stripped off the user name before authentication with AD.

CSCuj95908: ISE does not allow domain stripping for AD external store

There is a related recent thread on this that provides more information

https://supportforums.cisco.com/thread/2254662

[Charlie Moreton]

This was introduced in Patch 4 for ISE 1.2, you must have recently applied the patch, which is why you are just seeing this for the first time.

Documentation for this has not yet been added to the Release Notes, Installation Guide or Configuration Guide.