I have always looked at this question from the "failure" point of view. B31.1 and B31.3 consider primary (force driven) failure as the primary mode of failure. Then Markle published his papers in the early 1950s detailing another mode of failure - displacement driven. He referred to this mode of failure as secondary.

B31.1 and B31.3 (and similar codes) address these two failure modes as primary (what CAESAR II calls "sustained" and secondary (what CAESAR II calls "expansion").

I'm sure you can find better more detailed explanations of this topic in the various texts focusing on Pipe Stress Analysis as well as the B31 Code.