MikroTik

I have my MT working with 2 interfaces ethernet, one public (conected to my Isp) and the other private (conected to my local net), and I´m doing src-nat.
My doubt is in the grafic which is in packet flow (in MT page), input interface is my public interface and output interface is my private interface or both (public and private) are output or input interfaces depending the traffic sense??? Please helppppppppppppp!!!

input and output are going to and from the router. forward is anything going _through_ the router. All interfaces can hit any of those chains.

Sam