The Zone AF is a “mini” version of the Automatic AF Point Selection, the default AF mode. It works in the same way as the Automatic AF point selection except that it restricts the AF points to a small cluster (zone) that is selected.
Within this zone, the system will focus on the closest subject with adequate details, even if you're in Servo AF Mode.
You can select from one of five zones: the nine-point cluster in the center of the viewfinder or one of the a four-point triangular cluster to the left, right, above, or below the center zone.