MapServer 4.6 requests can probably be broken down into the following steps. Here the common name is presented along with its corresponding code fragment and/or the name of the source file.

• Create a mapserver object using mapserv.c.loadParams and mapserv.c.loadMap
  
• Parse parameters mapserv.c.loadForm
  
• Determine the mode. One of three are know to me, browse,query, and nquery. This is determined by the msObj.Mode property. Most of this logic is in mapserv.c.main.

Only the Browse mode is explained here.

• Extent, scale, and query files are evaluated using mapserv.c.setExtent mapserv.c.checkWebScale and mapserv.c.msLoadQuery
• Images are generated using maptemplate.c.msGenerateImages
• Map is drawn using mapdraw.c.msDrawMap
• Map is drawn using mapdraw.c.msDrawQueryMap
• Legend is drawn using maplegend.c.msDrawLegend
• Scalebar is drawn using mapscale.c.msDrawScalebar
• Reference map is drawn using mapraster.c.msDrawReferenceMap
• 
  

NOTES : Nice things to have.

• Two maps on one page.