The ExtentedCoordSyst.txt file is used with the ​SQL Server Spatial provider (for SQL Server 2008).

The ExtendedCoordSys.txt is composed of the following information (it can be copied and pasted into the file directly as represent comments)

The format for each line in this file is:

<SRID> <Coordinate System Name> <WKT>

where:
SRID: is the SRID for each coordinate system. For all coordinate systems initially in the file, the SRID
is the coordinate system's EPSG number.

Coordinate System Name: the coordinate system name.

WKT: a string in Open Geospatial Well-Known Text format.

Only Geodetic or Projected coordinate systems can be listed so the WKT must start with either GEOGCS or
PROJCS.

Each field must be separated by spaces (sorry, no tabs).

Coordinate System Name can have embedded spaces (though this is probably unlikely). All words after the EPSG number,
up to before the WKT, are treated as part of the Coordinate System name.

The file must reside in the FDO com directory (A sub-directory named 'com' that is in the same directory as the
SQLServerSpatial provider DLL).