The ITU also looks after frequency and orbit registration details.
So the bottom level is the Administration, the next level is the regional grouping and the top level is the ITU, which is the oldest organisation within the United Nations, having been formed in 1865. It's because radio waves don't stop at national frontiers that international co-operation is required. For amateur radio, because we want the same frequency bands on a world wide basis, the amateurs have to co-ordinate their efforts through the International Amateur Radio Union (formed in 1925) and then individual national societies lobby their Administration - some do more than others. Then the IARU lobbies at both the Regional level and at the ITU itself, with Technical Consultants and Technical Representatives attending relevant meetings. In ITU, it's the Working Parties in Study Group 5, and Study Group 1. SG5 is concerned with Amateur, Aeronautical, Mobile, Marine services: SG1 is concerned with Spectrum Engineering. I have been attending SG1 and some of its Working Parties and Task Groups on behalf of IARU since 2001, and as amateur radio consultant to the UK delegation to a Task Group for six years before that.
Does that answer the question?
Yes it does, now I have a better picture of the relations between those organizations! Thanks!