Spectra Precision FAST Survey Reference Manual - Manual (Page 357)

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The Datum definition screen is where you select the ellipsoid and Helmert parameters to apply.
If you select Add User Defined within the Coordinate Projection List screen, you proceed to the screen below:
Here you enter the name of the new system and select the projection type and parameters, or you can “Load File” to load
a pre-defined system stored earlier (in the form *.sys or *.csl).
If you select “Add Predefined” to create a new, stored projection system, you will be presented with this screen: