The trajectories of ELMy H mode discharges inthe pedestal temperature-pedestal density plane form cyclescharacterized by high pressure and low pressureboundaries. While at least for type-I ELMs the high pressureboundary seems to be associated with the ballooning limit inthe pedestal, the low pressure boundary is poorlyunderstood. The scaling of the low pressureboundary is assessed for JET type-I H modes. The discussionis guided by a semi-empirical model which emphasizes therole of SOL physics. Our results suggest thatthe high pressure and low pressure boundaries are governedby distinct physics mechanisms.