That would usually be from zero gage pressure to the operating pressure and back. Not design pressure: See Fig 4-130.1 and you'll note that Sa (alternating stress) is compared against stresses derived from operating loads. It could also be from one pressure to another and back (say 100 psi to 200 psi and back to 100 psi). Part 5-110.3(g) explains how to evaluate multiple types of cycles (say 1000 cycles 0 to 100 to 0 psi and 50 cycles 0 to 200 to 0 psi).
