It's a complex concept born of frustration, anxiety and paranoia! When you get your next batch of chicks, say in two years when you wish to refresh the laying or meat stock, you will face the problem of integrating smalls with the bigs. The bigs will thrash the smalls unless they have a safe place to run to. There's also the issue of protecting their feed and water because the pecking order competition will cause the smalls to eat less than they need for growth and health until they grow to equal size. So the food needs to be kept in the safe area where the smalls aren't kept from it. This safe area can be a rather small temporary haven in a corner of the run or a fenced section of the run. Small portals are constructed into all sides of this chick safe pen that are 5" x 7" so only the chicks fit, and the bigs can't follow them.
On top of that, you will face having to integrate them into the coop for sleeping purposes. This is even more frustrating and crazy. The big problem is getting the chicks to learn to go into the coop at night when they've been growing up elsewhere, such as a brooder box in the house. So a dedicated portion of the coop with its own entrance into the "grow-out/safe pen" is a god-send. Then you will still have the same luxury you will be enjoying this first time, not having to have the chicks (and you) endure the ruthless bullying when you transfer them from the brooder.
There are all sorts of coop/run designs to accommodate a grow-out pen. I happen to have built an extra coop at one point when I had three belligerent roosters and wanted a bachelor quarters to keep the hens from being constantly harassed. Oh, oh. That's another problem you will be confronted with. Sorry.
Anyway, I was smart enough to build numerous entrances in that new coop so when the last roo died, I converted part of it into a chick coop and fenced off an area of the run adjoining it so it could be a chick grow-out pen. When the chicks reach adult size, the gate between it and the rest of the run is opened and all the chickens then have access.
I betcha thought this chicken biz was simple, huh?
