NSI files contains the scene description for rendering. NSI stands for Nodal Scene Interface1. The NSI It was created (2017-2018) as an open standard and modern version of the old RIB file (Renderman Interface Bytestream) we use used to support since the early 2000s. The complete specs for NSI specs are available
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 the an NSI file from binary encoded to ASCII ACCII, you can use the renderdl command utility:
...
To render an NSI file, you use the "renderdl" command command line utility. It reads a file containing scene description commands and “executes” them to produce an image.
...