Just looked it up online, and it refers to the number of whistles per minute that the pressure cooker gives off, which indicates it is an old style of cooker, using weights. The article recommends a more common sense approach. Identify the ingredient, check what setting the pressure cooker should be on, and cook for the time the recipe species, and at the pressure you have identified.

This is the link IO found that mentioned whistles   http://www.hippressurecooking.com/does-your-pressu.....e-whistle/

Hannah