You define the scope of settings with the Store Selector Control, which is always placed right above the settings. By default, you configure settings for all of your stores. If you want to configure a particular setting for just one of your stores, choose the target store and activate the override option for this setting. The control to activate the override option for a particular setting is always placed between the label and the input field for the setting. By entering a value for a particular store, you can override the global value of the setting that's defined for all configured stores.
Multistore Configuration: Store Selector Control
Multistore Configuration: Override Setting Control