The phrase "ultimate sovereignty" means having the highest power and authority over everything. It refers to having complete control and decision-making ability without anyone or anything above you. Full definition
If the government is crappy, final responsibility lies with those with ultimate sovereignty, ie. (nystateofpolitics.com)
In the first sense, the kingdom of God means the eternal, ultimate sovereignty of God. (religion-online.org)
To temper any concerns about the transfer of this power, the Government retained ultimate sovereignty over the setting of the rate and granted only operational control over the attainment of a centrally determined inflation target. (politics.co.uk)