What is parameter `object` in NSNotification addObserver:?

One of my class named Message.m is posting a notification with an object sentObject as below

NSDictionary *sentObject = [NSDictionary dictionaryWithObjectsAndKeys:draftData.arr,@"data", nil];

//Post notification to inform a receiver to reload data     
[[NSNotificationCenter defaultCenter] postNotificationName:@"reloadDuringSave" object:self userInfo:sentObject];

DraftData.m will be be the receiver to catch the notification as follow

[[NSNotificationCenter defaultCenter] addObserver:self 
                                         selector:@selector(dataReloaded:) 
                                             name:@"reloadDuringSave" 
                                           object:nil];

For posting notification, userInfo can be nil or can be an object (like sentObject as type of NSDictionary in this example).

Question:

What are other params for object in addObserver method? Can they be anything other than nil, and if so what?