PREN stands for the pitting resistance equivalent number - a value calculated from the alloy composition that indicates its typical resistance to pitting corrosion. The industry specifications NACE MR0175 and NORSOK M-630 require the three main super duplex stainless steels - Ferralium® 255, UNS S32750 and UNS S32760 - to have a PREN above 40.
PREN can vary from production heat to production heat, given the slight variability in the elements used in the calculation, namely chromium, molybdenum and nitrogen. The specifications of the individual super duplex grades also differ.
The PREN formula does not account for elements such as copper, which is proven to improve the corrosion resistance of Ferralium® 255 in various acids. A threshold of PREN above 40 is therefore a sensible target. For how the figure is calculated, see Understanding PREN.