The AF-area modes are designed to give you control over how many AF sensors are in use at one time, and different ways to track subject movement.
Figure 4 shows the AF-area mode selector in the Single-point AF position.
This mode uses a single AF sensor out of the array of 51 sensors to acquire a good focus. As mentioned before, you can control which sensor by selecting it with the Multi Selector. In Figure 5 notice how the center AF sensor point (of the 51 sensor points) is the one that provides focus information.
If I have two people standing next to each other, but with a gap in the middle, I'll do one of two things.