CAPPluginReturnWatch and CAPPluginReturnSync

When writing a plugin in iOS, I see the native function can return one of these types as defined in CAPBridgedPlugin.h. Here are all of the return types:

#define CAPPluginReturnNone @"none"
#define CAPPluginReturnCallback @"callback"
#define CAPPluginReturnPromise @"promise"
#define CAPPluginReturnWatch @"watch"
#define CAPPluginReturnSync @"sync" // not used

My questions are:

  1. What is CAPPluginReturnWatch and how does it work?
  2. Why does CAPPluginReturnSync have the “not used” comment beside it?