RON is simply the petrol's resistance to pinking/detonation/pre-ignition (call it what you like). Modern engines adapt ignition timing to be as advanced as possible (more power, better efficiency) until the engine management detects pinking and then backs off a bit.
But, engines also have non-adjustable settings that also affect pinking (like compression ratio). So it may well be on a low-performance engine that the ECU has set maximum ignition advance on 95 RON fuel. 99 would be no use. Likewise a high performance engine (higher CR) might need 99 RON fuel to get the optimum advance, and so 95 RON will cause it to go into low power mode.
I'm going to guess that the Civic is somewhere in between with a CR of 10.8, and 99 RON
might be useful (but then it might not). My 968CS
had to run on 99 RON and it had a CR of 11.1. Try it and see!
More reading:
http://en.wikipedia.org/wiki/Compression_ratio
http://www.faqs.org/faqs/autos/gasol...section-1.html
And in answer to "Are different petrol bands different?", this is old, but still a good read:
http://www.faqs.org/faqs/autos/gasol.../preamble.html