There can not be a cross contamination or sucking, siphoning etc. of that sewage water with the use of an air gap.
And "Tom" says the codes now allow either an air gap or an air break; doesn't matter, your choice. I guess the code guys don't see much chance of siphoning with an air break, personally I don't either and I think if you have sewage overflowing onto your floor you that's more important than if it touches the end of your softener's drain line but... except in California as dittohead mentioned, only an air gap is allowed.
Terry mentioned only an air gap and mentioned siphoning, I asked him about what "Tom" said about an air break being allowed and an air break not preventing the siphoning.
It seems that you may not know the difference between the two or, you can't follow the conversation or... you're just playing dumb. I go with the latter but maybe I'm wrong and it is the second choice.