pam.d for authentication

Hi Experts,
I am new to PAM configurations and trying to understand the differnece between below two. I tried google and man but didn't completely understand. Please let me know what is the difference in the way pam authenticates between the two below.
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 more passwd
#%PAM-1.0
auth       required     pam_stack.so service=system-auth
account    required     pam_stack.so service=system-auth
password   required     pam_stack.so service=system-auth
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more passwd
#%PAM-1.0
auth     include        common-auth
account  include        common-account
password include        common-password
session  include        common-session
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Thanks in advance.