Which book and where does it talk about the social discounts you get from putting dots into Status:Invictus?

In the V:TR core rulebook on page 91.

I forget where it says it but the reasoning for the discounts (if it matters) is that the Invitus are the Leaders of the WOrld, and by contacting someone who knows somebody, they help get you the stuff easier with a harsh word/command or a quick bit of praise towards your reptuation.

It explains it on the page.

It also explains the Carthian discounts for Herd, Allies, and Contacts by explaining how the Carthians have much closer ties to the mortals and the "working" class.