[GRASSLIST:6964] Ellipsoid definition

[Luca Penasa]

Listing the various ellipsoids this is what i see:

...
rd18    Amersfoort
                        (bessel ellipsoid)
---
rome40  Monte_Mario
                        (international ellipsoid)
---
S-42    Pulkovo_1942
                        (krassovsky ellipsoid)
...


I cant understand whats the meaning for the ellipsoid into the brackets. 
Like (international ellipsoid) near to rome40  Monte_Mario. I know that 
the rome40 ellipsoid is based on the international one, but i cant 
understand why have I to specify at this point that im going to use the 
rome40 ellipsoid, when after i'll have to specify the towgs84 parameter. 
why grass need this two different definitions? and... how grass built 
the rome40 ellipsoid from the international one?
 
 
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