Earth Reference Models
ERMs are ellipsoids of revolution,for example WGS-84.
They are homogeneous, smooth, may not have the same centers, may not have parallel axes, their surface is not mean sea level, and they are NOT the Earth.
ERMs are not coordinate systems, but can be used to define a system.
SEDRIS allows for 21 common ERMs.
For more information try “Geodesy for the Layman” at http://164.214.2.59/geospatial/products/GandG/geolay/toc.htm.