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Please tell the difference between setCallBack and getCallBack function? Also, If possible can someone please tell me why what is .this function and what does this code is actually doing. (Its related to Instant Notification App https://trailhead.salesforce.com/content/learn/projects/workshop-platform-events/platform-event-subscribe)

const callback = function (message) 
 console.log('Event Received : ' + JSON.stringify(message));
 helper.onReceiveNotification(component, message);
 ;

 // Subscribe to the channel and save the returned subscription object.

 empApi.subscribe(channel, replayId, $A.getCallback(callback)).then($A.getCallback(function (newSubscription) 
 console.log('Subscribed to channel ' + channel);
 component.set('v.subscription', newSubscription);
 ));

action.setCallback() is used to provide a callback function when making a call to a server-side Apex controller:

 var action = component.get("c.someMethod");
 action.setParams( param: cmp.get("v.aParameter") );
 action.setCallback(this, function(response) 
 // Do things.
 
 $A.enqueueAction(action);

$A.getCallback() is used when passing a callback function to a function outside the Lightning framework that might interact with a component outside the normal rendering lifecycle, such as setTimeout():

window.setTimeout(
 $A.getCallback(function() 
 cmp.set("v.visible", true);
 ), 5000
);

You do not need to use $A.getCallback() when interacting with Apex server controller methods.

The code that you posted is using the <lightning:empApi> component to subscribe to a Platform Event. The subscription is handled by a JavaScript library that speaks the CometD protocol and callbacks are performed asynchronously at any time. Hence the use case falls into the realm of $A.getCallback() as described above.