User Principal Name handling rule - Synchronize option is greyed out in intraforest scenario</ (4246929)

On the "Set security options" tab in synchronization properties under User Principal Name handling, the Synchronize option is greyed out. Why is it not possible to select it?

The synchronize option is greyed out in intra-forest synchronizations, since the userprincipalname attribute must be unique within the Active Directory forest. Synchronizing this attribute between two different objects within a forest may cause issues and is not recommended. As per http://msdn.microsoft.com/en-us/library/ms677605%28v=vs.85%29.aspx

"The UPN must be unique among all security principal objects within the directory forest."

This behaviour is by design, configure Migration Manager synchronization properties so that UPN suffixes are different between source and target domains.