It depends on your single source layout. There is the RHWND.APJ file but I think this only has non-WebHelp based window definitions.
*.ppf holds window definitions for non-WebHelp. WebHelp windows are stored in rhwnd.apj.
Strangely enough, the .ppf is not referenced in the XPJ as a collection. A bit of divergence from RH’s regular inner workings.
The layout I need it for is the html help (chm) output. I had assumed that it was the hhp file because, well, hhp traditionally was the project file if you were only using Microsoft's s actual Html Help Workshop to generate a CHM. But I think RH uses the HHP for more than just chm layouts.
In any case, I doubt the hhp is the source file that holds the window definitions, because I think the hhp can get regenerated if deleted.