Workflow Manager | Use of Subscribe Activity

Hi 

I am new to workflow manager and exploring the use of pub/sub acitivities. What I have understood is to "Receive Notification" activity is some what similar to "Receive and Send Response" activity WF4.5. However, I did not understand the use of "Subscribe" activity. I have downloaded, pub/sub sample activities from MSDN Using Pub/Sub Activities. 

In this sample "Subscribe" activities has been used in "OrderTally.xaml" but not in "ProcessOrder.xaml". Can anyone please explain  when to use "Subscribe" activity? Do we always have to use Subscribe with Receive Notification activity? 

Regards,

RK

Radhakrishna