The RAYHOSTS file is 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 Scene 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.

The RAYHOSTS file is named max.rayhosts. By default, it is located in the \mentalray directory inside the program 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.