This is a PPWIZARD command line switch. You can set up your own default switches in the "PPWIZARD_OPTIONS" environment variable.
This command sets variables (like #define), but from the command line. You can set as many variables as you wish.
If the contents needs to include a space you should use "<?Space>" instead.
Define 2 variables
ppwizard 1.in /output:out\*.out /CrLf /Define:OpSys=OS/2
In your code you could do:
#if '<$OpSys>' = 'OS/2'
...
#elseif
...
#endif