Allowable Stress Amplitude in ASME BPVC Code Section VIII Div. 2

In ASME BPVC Code Section VIII Div. 2

Primary menbrane + Bending stresses + Secondary Stresses + Peak = Sa 

What's the relationship between the above Sa and the Stress Amplitude in Fatigue cycle curve? Is that a 1:2 or a 1:1 ratio?

Any comments will be appreciated.