Inner rust

Outer rocker from front fender to door tapers in to form an inner rocker at rear of door and 1/4 panel sits over top with a gap to let water drain. I have seen ppl cut that out form new metal and weld it in.
So you are saying that if I were to buy the outer rocker that piece that is rusted in my picture would actually be part of that? I've been searching for some pics online but couldn't find anything clear. Thank you.