/Fi (Preprocess Output File Name)
Specifies the name of the output file to which the /P (Preprocess to a File) compiler option writes preprocessed output.
Use the /Fi compiler option in combination with the /P compiler option.
If you specify only a path for the pathname parameter, the base name of the source file is used as the base name of the preprocessed output file. The pathname parameter does not require a particular file name extension. However, an extension of ".i" is used if you do not specify a file name extension.