It depends on the sump "style". if you go refugium it's best to have it before that section so you don't skim out your pods
that will grow in there.
A berlin style I believe is just a first chamber input, middle chamber holds the skimmer and the 3rd chamber has the return pump.
Here's another link that might help
Berlin method of filtration
Refugiums,fuges or refuge
I wanted a refugium to grow out pods
so thats the way I went. On my upcoming 90g I will be using a wet/dry sump with skimmer in the first section as I will be doing a fish only tank.
I wish I could be more specific and give you better links but it's what I got.
If you have a specific question just ask away. I've gotten pretty good with sumps and there so many people here who are really really good.