IPMF and PMF
My understanding is that PMF is that actual discount that was applied when pricing, i.e. if liability coverage alone would be $100 and property coverage alone would be $100, and if PMF is .9 (or 10% credit), then the insured would pay $180. In contrast, the IPMF is an artificial number created to make the loss experience of the two groups equal. For example, if you are analyzing loss experience and find that those who purchase monoline liability coverage have a pure premium of $100 and those who purchase liability coverage as part of a package have a liability pure premium of $85, then you set the IPMF to .85, regardless of what was actually charged in the past.
I am sure this explanation oversimplifies what is actually done, since the article refers to a section called "minimum bias procedure", which is not on the syllabus and my study guide from NEAS says "candidates rarely understand". So, I'm hoping this high level view is enough.