NSI files contains the scene description for rendering. NSI stands for Nodal Scene Interface1. It was created (2017-2018) as an open standard and modern version of the RIB file (Renderman Interface Bytestream) we used to support since the early 2000s.
1 For the technically inclined, one can read the full NSI 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 an NSI file from binary to ACCII, you can use the renderdl command utility:
renderdl -cat binary.nsi.gz > ascii.nsi
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