Specification of an orbital system, in which orbital elements are defined. Systems can be defined around all major planets and barycenters (and Sun, Moon, SSB..). They may be referenced to the GCRS, mean, or true equator or ecliptic of date, or to a plane that is tilted relative to that.
For example, The Minor Planet Center (MPC) publishes up-to-date orbital elements for asteroids and comets, which are heliocentric and referenced to the GCRS ecliptic. Hence 'center' for these is NOVAS_SUN, the plane is NOVAS_ECLIPTIC_PLANE and the type is NOVAS_GCRS_EQUATOR.
The orbits of planetary satellites may be parametrized in their local Laplace planes, which are typically close to the host planet's equatorial planes. You can, for example, obtain the RA/Dec orientation of the planetary North poles of planets from JPL Horizons, and use them as a proxy for the Laplace planes for their satellite orbits. In this case you would set the center to the host planet (e.g. NOVAS_SATURN), the reference plane to NOVAS_EQUATORIAL_PLANE and the type to NOVAS_GCRS_EQUATOR (since the plane is defined by the North pole orientation in GCRS equatorial RA/Dec). The obliquity is then 90° - Decpole (in radians), and phi is RApole (in radians).
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