Hello Yamini
Rene’s lesson on types of stub areas very clearly indicates the functionality of each. You can find it here.
However, I’ll try to give you a quick summary of why you would use each case in a real life scenario.
A stub area would be configured if a network segment had only one choice for routing all traffic. An example of this would be a branch office that has only one gateway. There is no need for LSAs of type 5 to be advertised within this network since all routes will point to the local default gateway.
A totally stubby area would be configured if you have additional OSPF areas in your network which means that Type 3 LSAs would also be sent among OSPF routers. The totally stubby area would also block Type 3 LSAs since they too would be routed via the one and only gateway available to the network segment.
A Not So Stubby Area (NSSA) is defined as a stub that includes an ASBR to another routing protocol autonomous system. In this case, you do not require Type 5 LSAs since there is only one route to other OSPF areas. So you would configure an NSSA if you have an additional “way out” of the network, but via an ASBR to an AS of a different routing protocol.
I hope this has been helpful!
Laz