When I first learned about Positional Astronomy (50 years ago), everything was described with respect to the Equator and Ecliptic (true or mean, of date or of a specific epoch). Then 'they' invented the International Celestial Reference System (ICRS). An inertial reference system, so true and mean no longer apply and neither does it require an epoch to be given.
Now I see things are referred to the International Celestial Reference System Version 2 (ICRS2).
What changed?
Not much has changed. Each update is consistent with its predecessor.
The original ICRF is defined by 212 extragalactic sources. The ICRF2 is defined by 295 sources, 97 of which are shared by ICRF1
The reason for the change was greater accuracy. The 295 sources in ICRF2 allow for locations accurate to less than 40 micro-arcseconds (µas) compared with 250 µas for ICRF1. The ICRF2 is consistent with ICRF1, to within the latter's precision.
Both reference frames agree with the J2000.0 frame, to within the precision of the J2000.0 frame. The ICRF is several orders of magnitude more precise than can be achieved by optical astronomy.
