The mental ray renderer uses several file formats to save information.
Final Gather Map file (.fgm). Saves the results of a final gathering pass. Generating and saving an FGM file can speed up subsequent renderings.
mental images scene description file (.mi).
You can generate a text (ASCII) MI file that stores a scene description. A mental ray renderer can use this file to render
your 3ds Max scene. (There is a binary MI format as well. The mental ray renderer in 3ds Max does not generate this format.) The exporter always generates mi3 (mental ray version 3) format. It does not support mental
ray version 1 (mi1). You specify a name and location for the MI scene description on the Render Setup dialog 
Processing panel Translator Options rollout.
See the chapter “Scene Description Language” in the mental ray Manual for a complete description of the mental ray scene description language.
mental ray rendering pass file (.pass). Saves the result of a single mental ray rendering pass. You can create a final rendering by merging multiple passes. The PASS file format includes Z-buffer information to aid in merging passes.
Controls for creating and merging PASS files are on the Render Setup dialog Processing Panel Translator Options rollout.
mental ray photon-map file (.pmap). A binary file that the mental ray renderer uses to generate the effects of caustics and global illumination. You specify a name and location for the PMAP file on the Render Setup dialog Indirect Illumination panel Caustics And Global Illumination rollout.
mental ray hosts file (must be named max.rayhosts). A text (ASCII) file that lists the name of host systems capable of mental ray rendering in a distributed network. You turn
on distributed rendering on the Render Setup dialog Processing panel Distributed Bucket Rendering rollout.
Each line in the RAYHOSTS file contains the name of a host system. The host name can be followed by a semicolon and a port number of the service to connect to. The host name with or without the port number can also be followed by white space and a list of mental ray command-line parameters.
See the manual, Rendering with mental ray, for descriptions of the mental ray command-line options.
When you click Add on the Distributed Bucket Rendering rollout and use the Add/Edit DBR Hosts dialog to add a host or satellite processor, this updates the RAYHOSTS file. So does clicking Remove to remove a processor.
By default, max.rayhosts is located in the \mentalimages directory in the 3ds Max folder. You can change the default location by creating an environment variable named MAX<X>_MI_ROOT, where <X> is the 3ds Max version number, and setting the variable's value to the path of the max.rayhosts file you want to use.
mental ray shadow map file (.zt). A binary file that the mental ray renderer uses to accelerate the generation of shadows. You specify a name and location
for the .zt file on the Render Setup dialog Renderer panel Shadows & Displacement rollout.
