From my readings, NSS seems to be a superset of PAM. PAM on the other hand are just limited to authentication/authorization. Am I correct ?
What is the difference between NSS and PAM?
authentication;pam
Yes, PAM is concerned with authentication and authorization but not identification:... you should know that PAM does not handle identification. Systems  that rely on PAM normally use Name Service Switch (NSS) to manage  identification. NSS also [like PAM] originated with Solaris, and  predates PAM by a few years. The lack of integration between NSS and  PAM has caused a continuous migraine for Unix-like systems ever since.Pam Mastery, Michael Lucas, chapter 0