A coordinate system is a service object for point, segment and polygon maps, and for georeferences (the latter are used by raster maps and will be treated in the next section).Vector maps and georeferences always need a coordinate system.
A coordinate system defines the possible XY-coordinates or Latlon-coordinates that can be used in your maps and thus stores information on the kind of coordinates you are using in your maps.You may for instance use user defined coordinates of a certain UTM zone.
A coordinate system may have information on the map's projecttion,ellipsoid and datum.