Nikon sidecar files are created during NEF processing within one of Nikon's processing applications such as ViewNX-i or its replacement NX Studio
NKSC files are in fact XMP xml files which contain additional Nikon specific tags. They are created during processing and placed in a folder called "NKSC_PARAM".
- ↑ https://exiftool.org/TagNames/Nikon.html#ast
- ↑ https://nikonimglib.com/nxstdo/onlinehelp/en/11_preference_18.html
- ↑ https://www.dpreview.com/forums/thread/4447605