... It's a bit more complicated than that - I haven't mentioned MON and PON .....
Typically AKI is 4 - 6 points lower than RON, so US AKI 91 fuel is the rough equivalent of our UK 95 RON.
...
Exactly that.
RON is Research Octane Number, MON is Motor Octane Number. The tests are done under different speed/load/temperature conditions. MON is typically 10 numbers lower than RON for a given fuel.
Just for info, the USA normal is to display (RON+MON)/2, so as Badger says typically 5 numbers lower than RON as shown in the UK/Europe.
In addition, fuels in different parts of the world
can behave differently even for a given octane number, I had particular experience in industry with issues in New Zealand with fuel characteristics not suiting our cars (or vice versa if you prefer). We couldn't repeat the problem back here with European fuel, we had to send engineers out there to address it "on site".
Sulphur (sulfur .. ) content can vary significantly, which can cause corrosion issues particularly with some plated cylinder bores.
When working at one consultants we had an early BMW DGI engine in for assessment. It had done around 30k km if I recall, and the inlet valves were heavily carboned, I did air flow rig measurements. I don't know what the latest DGI engines are like for this.