Coral, The NACE/ISO document is the one that should be used since it has replaced the NACE MR0175 2003 document. The NACE/ISO document has followed the work in EFC 16 regarding sour regions calculated by in situ pH and H2S partial pressure. The old NACE 2003 document only had partial pressure of H2S (or total system pressure if the pressure was low enough) and different graphs for vapour and mixed streams. If you use Annex C and D in the NACE/ISO with the graph in Figure 1, you will find yourself in one of four regions. There is not much distinction between materials in the three SCC regions, but some differences can be found for material requirements for certain product types.