You get to choose which pin goes to which relay/zone. If you have already installed opensprinkler firmware onto the Pi then you can select a station (click the associated cog). In the settings popup, click the Advanced tab and select “GPIO” as the station type. You can now select the gpio pin that you want to toggle when this station needs to water. Make sure you then wire up that gpio pin to the corresponding relay. You may need to swap the Active High/Low setting in the Advanced options depending on your particular relay board.