Fatigue life assessment using FEA results

Hello all, I am new to repair validation of compressor stage fan blades. To validate, I have to model the repair and compare the results with reference model results. As per the methodology document for fatigue life validation, we have to plot the stress values from linear static (on mean stress axis)and modal analyses (on alternating stress axis) of a haigh diagram. Similar plot is made for reference model and if variation in margin is less than 5%, we can report the repair is ok.
Here, what I don't understand is the usage of stress values from a modal analysis (yes,modal analysis stress is scaled down if it falls outside the curve).Can anyone help me out in understanding this. Thanks in advance