这种类的用法是什么原理？

看代码时碰到了这样一个问题
先粘上相关的部分代码

class CMOOSGeodesy
{
public:
    class CEllipsoid
    {
    public:
        CEllipsoid(){};
        CEllipsoid(int Id, const char* name, double radius, double ecc)
        {
            id = Id;
            ellipsoidName = name;
            EquatorialRadius = radius;
            eccentricitySquared = ecc;
        };

        int id;
        const char* ellipsoidName;
        double EquatorialRadius;
        double eccentricitySquared;

    };

};

//! Ellposid information for CMOOSGeodesy
static CMOOSGeodesy::CEllipsoid ellipsoid[] =
{//  id, Ellipsoid name, Equatorial Radius, square of eccentricity
    CMOOSGeodesy::CEllipsoid( -1, "Placeholder", 0, 0),//placeholder only, To allow array indices to match id numbers
    CMOOSGeodesy::CEllipsoid( 1, "Airy", 6377563, 0.00667054),
    CMOOSGeodesy::CEllipsoid( 2, "Australian National", 6378160, 0.006694542),
    CMOOSGeodesy::CEllipsoid( 3, "Bessel 1841", 6377397, 0.006674372),
    CMOOSGeodesy::CEllipsoid( 4, "Bessel 1841 (Nambia) ", 6377484, 0.006674372),
    CMOOSGeodesy::CEllipsoid( 5, "Clarke 1866", 6378206, 0.006768658),
    CMOOSGeodesy::CEllipsoid( 6, "Clarke 1880", 6378249, 0.006803511),
    CMOOSGeodesy::CEllipsoid( 7, "Everest", 6377276, 0.006637847),
    CMOOSGeodesy::CEllipsoid( 8, "Fischer 1960 (Mercury) ", 6378166, 0.006693422),
    CMOOSGeodesy::CEllipsoid( 9, "Fischer 1968", 6378150, 0.006693422),
    CMOOSGeodesy::CEllipsoid( 10, "GRS 1967", 6378160, 0.006694605),
    CMOOSGeodesy::CEllipsoid( 11, "GRS 1980", 6378137, 0.00669438),
    CMOOSGeodesy::CEllipsoid( 12, "Helmert 1906", 6378200, 0.006693422),
    CMOOSGeodesy::CEllipsoid( 13, "Hough", 6378270, 0.00672267),
    CMOOSGeodesy::CEllipsoid( 14, "International", 6378388, 0.00672267),
    CMOOSGeodesy::CEllipsoid( 15, "Krassovsky", 6378245, 0.006693422),
    CMOOSGeodesy::CEllipsoid( 16, "Modified Airy", 6377340, 0.00667054),
    CMOOSGeodesy::CEllipsoid( 17, "Modified Everest", 6377304, 0.006637847),
    CMOOSGeodesy::CEllipsoid( 18, "Modified Fischer 1960", 6378155, 0.006693422),
    CMOOSGeodesy::CEllipsoid( 19, "South American 1969", 6378160, 0.006694542),
    CMOOSGeodesy::CEllipsoid( 20, "WGS 60", 6378165, 0.006693422),
    CMOOSGeodesy::CEllipsoid( 21, "WGS 66", 6378145, 0.006694542),
    CMOOSGeodesy::CEllipsoid( 22, "WGS-72", 6378135, 0.006694318),
    CMOOSGeodesy::CEllipsoid( 23, "WGS-84", 6378137, 0.00669438)
};

然后调用的时候是这样：

double a = ellipsoid[23].EquatorialRadius;

c++刚学不久，这是什么原理？