Nikon Sidecar
From Just Solve the File Format Problem
Nikon sidecar files are created during NEF processing within one of Nikon's processing applications such as ViewNX-i or its replacement NX Studio
File Format
NKSC files are in fact XMP xml files which contain additional Nikon specific tags[1][2]. They are created during processing and placed in a folder called "NKSC_PARAM"[3].
