The .cpb file describes the search capabilities of a source. The .pmf is the pre-mapping file that allows administrators to map search/index attributes that apply before the Muse ISR stylesheet is used to format the search for the source (thus "Pre"-mapping).
The .cpb file lists all the search indexes and Boolean operators which are supported by the Source (whatever the actual, native syntax of the attribute). It is also a place where other source capabilities could be added later for easy import into the Muse InfoBase. The search limiter sets are one example.
The .pmf file provides a mapping from one attribute in ISR to one attribute in translator. The .pmf mapping is TO an attribute which is handled into the ISR translator. All of the attributes handled (supported) in the ISR translator have been automatically extracted and recorded into the appropriate section in .cpb file.
By default, the PMF files map unsupported attributes to keyword.