Points are allocated unique geographical co-ordinates. These are latitude, longitude and height. The height could be either orthometric (above the geoid (defined as mean sea level)) or above the surface of the reference spheroid (ellipsoidal height). In order to map these co-ordinates, they need to be projected onto the plane. In South Africa we use the Gauss-conform projection. The projection effectively projects the latitude and longitude coordinates onto a surface referenced by a particular longitude and the equator. The South African system is defined in belts of two degrees in longitude. Every odd meridian is the central meridian of reference (simply referred to as that "Lo" or Longitude of origin). The Lo.s are 17, 19, 21, 23, 25, 27, 29 and 31. Each two degree belt is therefore flattened with reference to that Lo. (ie. Lo.19 has a flat surface representing the area between 18 degrees east and 20 degrees east)
http://w3sli.wcape.gov.za/Surveys/Mapping/mapprojn.htm