The cpl_modelio.nml file sets the filename for the primary standard output file. Existence is largely historical. The cpl.buildnml.csh script actually generates the _modelio.nml files for all components.

modelio namelist

diri (char)

unused

diro (char)

unused

logfile (char)

the name of the logfile for the component. default="", required but set automatically by ccsm scripts