The pdxtblkres command uses file descriptors while it processes Xerox resources:

fd0 (STDIN)

If you do not specify a resource file name with the command, the command reads the deblocked resource from standard input.

fd1 (STDOUT)

If you do not specify a resource file name with the command, the command writes the blocked resource to standard output.

fd2 (STDERR)

The command writes messages from the blocking process to standard error.