Bruno, the setting you probably want is at the top of this page:

See the box at the top that says "Sync Qantize" and is set to Off

If you touch this, you can set this to a note spacing in tiny units of time... I almost always use 480.

Then I turn on "Arp Play Only" which means my poorly timed key inputs aren't sounded...

Once set these two are set, whenever you change the notes you're touching on the keyboard, the arp waits until the next unit of time to sync that input (quantise it) to the currently playing arp time. This makes me a legendary arpist, such that I still play wrong notes sometimes, but my timing is never wrong, because it's quantised by this.

Sometimes I even forget that this setting is all about switching arps (Example: switch from ARP 3 to ARP 7). It's a way to quantize those arp switching buttons.

I think half the time I'm dealing with arp programming I set this thinking it has something to do with re-trigger quantize -- which it isn't. It ends up being inconsequential due to how I use arps -- but good to get a reminder about what this parameter is all about. The meaning doesn't jump out at you without doing some research.

Thanks a lot for the answers and comments: it does indeed clarify lots of things to me.

My original goal was the following: I am using one given ARP, and from time to time, I change the chord. I have realized that depening on the exact moment I press the new chord, the transition is more or less nice, and I was looking for a way to make this more robust (to the human timing errors). And I believe the actual answer to my goal is that of Andrew about "Sync Quantize". I'll check this on my synth as soon as I can and come back if needed.

Thanks to everybody.