Visual assessment/dig a hole and fixed depth and return period have the same data inputs in OverseerFM and are reflected the same way in the model.
Both you can chose to use the default setting for the irrigation system, or override these with a fixed depth per application and return period (FF).
A fixed depth and return period (FF) is used when the irrigation management strategy is based on visual assessment. It was considered that visual assessment was not reliable enough to be able to model a variable management strategy (i.e. one where either depth per application or return period or both are managed according to soil moisture status).