Usually I use the first one to define const, but I don't know the difference of the following clearly.
static NSString* kFetcherCallbackThreadKey = @"_callbackThread";
static NSString* const kFetcherCallbackRunLoopModesKey = @"_runLoopModes";
NSString* const kFetcherRetryInvocationKey = @"_retryInvocation";
static const NSUInteger kMaxNumberOfNextLinksFollowed = 25;