Portals are a very important tool to speed up the rendering of levels. Well defined sectors can greatly speed up rendering and allow various tricks, to understand what they are, read this guide.
CS uses portals to connect different sectors. A portal 
  can be for example a door or a window that connects the interior of an house 
  to the exterior. In that case the interior will be one sector, the exterior 
  another one.
  
  To add a portal to a 3dsmax scene follow this guidelines:
  Please note than the exporter will produce 1 sector per exported scene , so 
  if you want to make a level with many sectors: 
Next: Samples