The simple answer to this question is they should be used when the owner selects the use of these rules for a particular piping system. Note that the definition of High-Pressure Fluid Service simply requires that the owner specify use of Chapter IX. However, it is probably useful to understand why such a selection may be made.
Some guidance is provided in K300 (a), which states that "High pressure is considered to be pressure in excess of that allowed by the ASME B16.5 PN 420 (Class 2500) rating for the specified design temperature and material group." This is not a requirement, and the base Code may be satisfactorily used at pressures higher than ASME B16.5, PN 420 (Class 2500). However, the base Code rules become increasingly conservative and, in fact, impossible to use as the pressure approaches the allowable
stress (including quality factors). When this occurs, the wall thickness of straight pipe designed to the base Code becomes equal to the pipe outside radius, so the pipe would have no bore! It would be bar if designed to the base Code. By the same token, Chapter IX may be used at lower pressures; it has been used with high-strength steels with pressures as low as 28,000 kPa (5,000 psi).
A higher allowable stress, with a different pressure design equation can be used when the rules of Chapter IX are followed. As such, the wall thickness can be less. However, the tradeoff is that extensive additional and/or more stringent design/analysis, material testing (e.g. all material is required to be impact tested), examination and leak testing is required. Further, there are few engineering design firms that a properly qualified to design such systems.
The rules of Chapter IX should generally be used when there is sufficient economic justification based on reduced wall thickness to offset the other increased costs. Further, it is used when is becomes simply impractical to use the base Code because of the pressure design equation. Some consideration should also be given as to whether you will be engaging an engineering firm with the proper knowledge and experience to design the piping system using the Chapter IX rules.
Chapter IX bases the allowable stress on 2/3 of the yield strength of the material and gives no consideration to the tensile strength. As a result, considerable thinner piping systems can be designed using Chapter IX when the material has a high yield strength to tensile strength ratio. As a result, its use has been considered for piping systems to be constructed with high yield strength steel with pressures as low as 28,000 kPa (5,000 psi).
There is not a hard line at Class 2500, as some people assume because of the definition included in ASME B31.3, but rather it is a decision to be made by the owner, giving consideration to the economics. Both the base Code and Chapter IX result in safe piping systems, regardless of the pressure. It just becomes impossible to use the base Code when the pressure approaches the allowable stress (including quality factors).

