This is a PPWIZARD command line switch.
When ppwizard looks for a file (such as when it encounters a #include command) it searches in certain locations. This switch allows you to specify your own locations which will be searched before ppwizard looks in the standard ones.
If you don't supply a parameter ppwizard will forget (drop) any locations you have previously specified otherwise you are supplying a a list of items separated by ";" (or colon for unix). If the item begins with a "*" character then what follows is the name of an environment variable whose contents should be searched otherwise it specifies a directory.
It is a common requirement that the current directory be searched first if you need this to be searched before your supplied paths then you need to tell ppwizard this!
Once PPWIZARD is started you can also use IncludePath() to update the search path.
ppwizard.rex 1.in /IncludePath /IncludePath:.;c:\second;c:\third /IncludePath:c:\fourth