* This refers to the International Celestial Reference System
, which is the standard celestial coordinate system centered at the barycentre of the Solar System, with axes that are fixed with respect to objects in far-reaches of the cosmos.
A Reference System, particularly a Celestial Reference System, is a set of prescriptions and conventions together with the modelling required to define at any time a triad of axes.
The final result was the FK5  (there was also a FK6 , but it was never used as a materialization for any Celestial Reference System).
It was for these reasons that the International Astronomical Union (IAU) decided in 1997, at its 23rd general assembly, that the IAU Celestial Reference System was the International Celestial Reference System (ICRS) from January 1, 1998, in replacement of the FK5.
* correction due to proper motion (International Celestial Reference System to Barycentric Celestial Reference System transformation)
* generalized Lorentz transformation (Barycentric Celestial Reference System to Geocentric Celestial Reference System transformation including corrections due to annular parallax, light deflection, aberration)
The primary objectives of the IERS are to serve the astronomical, geodetic and geophysical communities by providing the International Celestial Reference System (ICRS) and its realization, the International Celestial Reference Frame (ICRF), the International Terrestrial Reference System (ITRS) and its realization, the International Terrestrial Reference Frame (ITRF), Earth Orientation Parameters (EOP) required for study Earth orientation variations and for transformation between the ICRF and the ITRF.
INTERNATIONAL CELESTIAL REFERENCE SYSTEM (ICRS) AND INTERNATIONAL TERRESTRIAL REFERENCE SYSTEM (ITRS)
Version 2.0 of the A catalog remedied that situation in 1998, being based on the same International Celestial Reference System
(ICRS) used by Hipparcos.
This authoritative source contains a wealth of information on spherical astronomy, time and calendars, celestial reference systems
, eclipses, orbital and physical ephemerides of the Sun, Moon, planets and satellites, and much more.