| IRegisterCallback | Callback interface used to notify the result of extension and source registration |
| HostAppWidgetInfo | The widget info class describes a host application widget. |
| InputInfo | The input info describes a host application input. |
| RegisterExtensionTask | Perform extension registration or update in background |
| DisplayInfo | The display info describes a host application display. |
| KeyPadInfo | The key pad info class describes a host application key pad. |
| DeviceInfo | The device info class describes a host application device. |
| DeviceInfoHelper | This class contains information about the watch |
| RegistrationAdapter | The registration adapter is used to access the registration content provider. |
| RegistrationInformation | Returns information needed during extension registration |
| RegistrationInformation.WidgetClassList | Class containing a list of widgets. |
| WidgetContainer | The widget container describes a host application display's widget support. |
| HostApplicationInfo | The host application class contains information about a host application. |
| TapInfo | The input info describes a host application input. |
| RegistrationHelper | The registration helper class helps an extension to register with host application based on the information provided in RegistrationInformation. |