The new ASME B31.3 2014 adds the nomenclature for SL for Equation (1b) as follows:
Para. 302.3.5 Limits of Calculated Stresses Due to Sustained Loads and Displacement Strains,
(d) Allowable Displacement Stress Range, SA.
The computed displacement stress range, SE, in a piping system (see para. 319.4.4) shall not exceed the allowable displacement stress range, SA (see paras. 319.2.3 and 319.3.4), calculated by eq. (1a)
SA= f(1.25 Sc + 0.25Sh) (1a)
When Sh is greater than SL , the difference between them may be added to the term 0.25Sh in eq. (1a).
In that case, the allowable stress range is calculated by eq. (1b)
SA= f[1.25(Sc + Sh) – SL] (1b)
For eqs. (1a) and (1b)
...
SL = stress due to sustained loads; in systems where supports may be active in some conditions and inactive in others, the maximum value of sustained stress, considering all support conditions, shall be used.
This explanation is followed by Appendix S example 2, and it means that Hot SUS is the basis for the evaluation of EXP(1b).
I think Eq(1b) shall be SE+ SE(P,W) ≤ f[1.25(Sc + Sh)] as secondary/peak stress evaluation (as a modified fatigue design by B31) where SE(P,W) is stresses due to pressure and content weight, exclusive non-variable loads such as pipe metal weight and permanent loads between installation and operation, but this modification may significantly impact for the current conventional way.