Functionally and mechanically, there is no difference. Assuming it's the same with the 1299 Superleggera, there are a very limited number of XXX SLV4 bikes that were used for promotion and press usage. I imagine a few of them have no mileage on them at all. For example, at the SBK and MotoGP Experiences that preceded, WDW2022, there were four (that I know of) XXX bikes at Misano. Two were used on track for the experiences and two were used for display - one complete and one partially naked to show the tasty bits a little better. Afterwards, the bikes are made available to the public with the main difference being that they not part of the 500 limited bikes. All of the race/track kit that was included with the numbered bikes is included with the XXX bikes. FWIW, I've seen 3 of the supposedly 4 1299SL XXX bikes that came to the USA (and 1 of the 2 XXX prototype bikes) before they were started, and they appeared to be as new except for a slight coloring to the titanium pipes. Everything else was showroom new, and warranty was the same as if they were numbered bikes. From what I understand, the small quantity of XXX bikes got distributed around the world.
I'm assuming you're asking this as you have one available to you. The numbered bikes are no longer available, so if this is your situation, I'd jump on it if you have the chance.