This is a Windows system installed file with Windows File Protection (WFP) enabled.
There are 55 versions of syncui.dll in the wild, the latest version being 6.3.9600.16384 (winblue_rtm.130821-1623). It adds a context menu to the Window's shell named BriefcaseMenu. The average file size is about 260.85 KB.
In Microsoft Windows, the Briefcase is a special folder that supports simple two-way file synchronization between itself and another folder. The Briefcase is designed for mobile PC users so that they may transfer it to a removable drive and have it synchronize with the computer to which the removable drive is attached.
(Note, the behaviors below are for all versions of syncui.dll, select a unique version for details.)
Located in the registry at 'SOFTWARE\Microsoft\Windows\CurrentVersion\Shell Extensions\Approved'
United States installs about 41.27% of Windows Briefcase.