Geographic data for any particular area is stored in separate layers. For example, roads are stored in one layer, parcels in another, and buildings in a third. To enable the data in each layer to integrate when displayed and queried, each layer must reference locations on the earth's surface in a common way. Coordinate systems provide this framework. They also provide the framework needed for data in different regions to be referenced in different ways. Each layer in the geodatabase has a coordinate system that defines how its locations are georeferenced.
In the geodatabase, the coordinate system and other related spatial properties are defined as part of the spatial reference for each dataset. A spatial reference is the coordinate system used to store each feature class and raster dataset, as well as other coordinate properties such as the coordinate resolution for x,y coordinates and optional z- and measure(m)-coordinates. If required, you can define a vertical coordinate system for datasets with z-coordinates that represent surface elevation. For an introduction to these properties, see The properties of a spatial reference.