If you play an Alien Swarm map, you see something like this:
If you're a mapper, you see something like this:
However, if you're an alien or a marine bot, you see something like this:
Today, I'll explain the third view. The one you don't see unless you're actually part of the game. The little yellow boxes (nodes) tell the AI "you can go here". The game automatically connects nearby nodes to form a node graph. It's like a street map, but there are no houses and streets can appear and disappear at any time if something blocks a path between nodes.
On the left is a room with the props removed. On the right is the same picture, but with the props. Open them in tabs and flip back and forth a few times if you can't connect them in your head.
There are two main problems with this layout: there are no hints for marine bots and some of the nodes are inside obstacles. For example, the forklift, dumpster, crates, and shelves are on top of some of the nodes. Those nodes are effectively nonexistent because they're always blocked.
In this picture, the marines have places to stand and the nodes aren't blocked by obstacles. Since the marines aren't allowed under the shelves or past the forklift, there are no positioning hints on the right. If there were, the marines could choose to stand there and try to walk where they can't. This is an example of a good layout for AI nodes.
In a game like Swarm Director 2 that depends heavily on AI, a good node graph is very important.