Maybe you want to use your own data bases with PP3. If they are not too large1 you may well do so. Of course, then you need to know the internal structure of the files. Although they are so simple that reverse engineering should be almost trivial, you find here a complete description of all of them.
Additionally I will give information where the original data of PP3's standard distribution came from.
The last point, the “label dimensions file”, isn't a real data base file, but an internal temporary file. It is mentioned here just for completeness.
 Unfortunately PP3 still has the disadvantageous behaviour of reading the whole data base file.