A zone is a section of your yard controlled by one of the numbered terminals on the bottom of the unit. For your yard, this would be defined as the section controlled by one solenoid/valve.
 
Information on the basic components of a zone.                                                                                                                                                                               

It may control many sprinklers but it is still a single zone.

A solenoid/valve will connect with the positive to the numbered terminal that you want to control it. The valve will have a common wire that will be placed in series with the rest of the group, or ran directly to either of the C terminals on the bottom of the unit.