A calculated value specific to stainless steels. It estimates the pitting corrosion resistance of a metal based on the amounts of crucial alloying elements. Specifically, the PREN values in the database are calculated as Cr + 3.3Mo + 1.65W + 16N, where each element is the average percent by weight. Higher values are more resistant.
The PREN is primarily an industry convention, and multiple variations exist. It is not uncommon to see the tungsten component omitted. In addition, it is sometimes calculated using minimum weight percentages instead of averages. This provides a more conservative value, but also a less detailed one (many stainless steels do not have a specified minimum nitrogen content). The particular calculation used in the database was chosen because it is used in ISO 15156-1.
There is actually some disagreement as to what "PREN" means. Some sources claim that PREN stands for "pitting resistance equivalent with nitrogen", which can also be written as PREN. In that case, its counterpart without nitrogen is the "pitting resistance equivalent" or PRE. The PRE is calculated in the same manner as the PREN, but without the nitrogen component. Since stainless steels contain very small amounts of nitrogen, the values of PRE and PREN are never far apart.
The same sources that differentiate between PRE and PREN also note that PRE can be used for all stainless steels, while PREN should only be used with austenitic and duplex alloys. However, since most ferritic and martensitic stainless steels do not have any specified levels of nitrogen, the distinction is mainly academic. It is entirely conceivable that the idea originated as PRE and PREN, and then evolved into the more common PREN.
PREN values can be used to compare different alloys. They can also be specified as a minimum acceptable value for certain applications. Some sources remark that alloys with a PREN of 40 or more can be called "superaustenitic", "super-duplex", and so on. However, the reader is reminded that "super" is a marketing term, not a technical one.