The name of the directory where Octopus stores information such as the density, forces, etc. requested by variable Output in the format specified by OutputFormat. This information is written while iterating CalculationMode = gs, unocc, or td, according to OutputInterval, and has nothing to do with the restart information.
output/output.F90 : 397
call parse_variable(namespace, 'OutputIterDir', "output_iter", outp%iter_dir)