NSI files contains the scene description for rendering. NSI stands for Nodal Scene Interface. The NSI was created (2017-2018) as an open standard and modern version of the old RIB file (Renderman Interface Bytestream) we use to support since the early 2000s. The complete specs for NSI specs are available here.
There are two kinds of NSI files: ASCII encoded and binary encoded. A binary NSI file is smaller than its ASCII encoded equivalent, but an ASCII NSI file has the advantage of being human readable and editable in a text editor.
To convert the file from binary encoded to ASCII ACCII, you can use the renderdl command utility :
renderdl -cat -binary myfile_ascii.nsi > myfile_binary.nsi
render
To render an NSI file, you use the "renderdl" command line utility. It reads a file containing scene description commands and “executes” them to produce an image.
renderdl myfile.nsi