The "Proximity state as Attacker" modifier is a modifier on warsupport that i have not seen explaned anywhere.

How is that modifier calculated during war ?

In most cases it seems to be -1 for the attacker, but in a recent war i have seen the ai geting +1 from it during a war they started (gaining war suport from it)

So it's clear that i don't understand that mechanic.

Would any one do me a favor, and explain how it works ?

Thanks for the explenation.

So when playing vs ai the pennalty won't be big,and can even give war suport.

But in multiplayer where everyone get's very simmular civic's, it will always be quite negativ

You can, AFAIK, each Civic shows which options other nations chose. Don't know if a clean UI that would show whole civic tree for each empire would be possible and it still wouldn't be taking into account event choices.