The SOM Precompiler is invoked as a consequence of the following NMAKE inference rule:
.csc.c: sc $<
This infers that to go from a .CSC file to a ".C" file the SOM Precompiler (sc.exe) will be invoked on the .CSC file ("$<").
Note that for the folder example, many default SOM Precompiler options are used. Readers may wish to investigate some of the other options that are available, by checking the IBM SOM Programming Reference.