CAESAR II, by default, uses the minimum S as the allowable stress for sustained loads. But you can choose other defined S's specified (by way of different temperatures)as your sustained stress limit. Find this entry on the Load Case Options tab of Static Analysis.
Sustained stresses may change because of load redistribution caused by pipe strain (e.g., lift off at resting supports). So since the calculated sustained stress is a function of temperature (because of pipe strain), you can associate a temperature for that state of "sustained" stress. And if you can associate a temperature with a sustained state, there is a related basic allowable stress at that temperature - Sh.
B31.3 302.3.5(c) states:
The sum of the longitudinal stresses due to sustained loads, SL, such as
pressure and weight in any component in a piping system (see para. 320), shall not exceed Sh, where Sh is taken from Table A-1 at the metal temperature of the operating condition being considered.