This is a PPWIZARD command line switch.
This command sets variables (like #define), but from the command line. This would typically be done to pass information to the script via the command line (possibly version number information or similar) removing the requirement to edit your source code.
You can set as many variables as you wish by using the switch once for each variable.
If the contents needs to include a space you should use "<?Space>" instead.
I recommend that if you use this switch you also ensure that you are also using the *CmdLine" dependancy type so that if you change the value of the switch then a rebuild will occur (look at generated dependancy files if unsure).
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