the pbs terminology is confusing: free could mean not entirely free, so partial is a better word (ie at least one job). 
internally, free maps to the torque built in ND_free state (0 or more jobs, but not all slots), partial maps to self defined ND_free_and_job (ie 1 or more jobs, not all slots filled). I'll make it clear in the help.
free nodes with 0 jobs are tagged idle.