If the application program is opening an existing file or creating a new file, you need to tell the runtime which file system to use. You accomplish this with one of two configuration variables: DEFAULT_HOST or filename_HOST.
filename_HOST filesystem
assigns an individual data file to a file system. Any file so assigned uses
the designated file system, and not the one specified by DEFAULT_HOST.
Substitute the base name of the file for filename. The name you substitute should not include any directory names and should not include a file extension.