Color values in Sun raster files are eight bits wide, so ppmtorast will automatically scale colors to have a maxval of 255. An extra ppmcscale step is not necessary.
The -s flag forces the result to be in RT_STANDARD form; otherwise, it defaults to RT_BYTE_ENCODED, which is smaller but, well, less standard.
The -x flag has to do with the color map. Normally, ppmtorast attempts to rearrange or add to the colormap so that white is in slot 0 and black is in slot 1 -- the standard Sun default colormap. However, if you are doing screenloads from within Xsun, on a CG2, you want the opposite -- black in slot 0 and white in slot 1. (Why would you be doing something this silly? Well, I have sometimes found it useful.) So, the -x flag sets this.
The pbmtorast filter is significantly faster if your images are just black and white.
Permission to use, copy, modify, and distribute this software and its documentation for any purpose and without fee is hereby granted, provided that the above copyright notice appear in all copies and that both that copyright notice and this permission notice appear in supporting documentation. This software is provided "as is" without express or implied warranty.