The original intention of B31.3 was to calculate a range stress under the combined loading of weight, pressure and thermal expansion (strain) between cold and hot and to limit it as Syc+Syh or (1.5*Sc)+(1.5*Sh), with an appropriate safety factor. Cannot say it is "operating" stress, it is a range. Same is true for EN 13480, with a different rule.

To simplify the calculation in B31.3 the range was considered for displacement stress only, as a subset of the range described above but setting conservatively the "sustain" to Sh.

"Liberal stress" means that, when calculate the allowable displacement stress range , you may remember how the original range was defined and consider there is a difference between the calculated sustained longitudinal stress and the allowable Sh.