[CentOS] pam_succeed_if

Hi,

The default system-auth file for PAM on CentOS has the following auth
section:

auth        required      pam_env.so
auth        sufficient    pam_unix.so nullok try_first_pass
auth        requisite     pam_succeed_if.so uid >= 500 quiet
auth        required      pam_deny.so

What's the use of the pam_succeed_if line? It will only be reached if
the pam_unix doesn't succeed and from my understanding it will prevent
system accounts from logging in. Is it useless or am I missing
something?

Thanks, Daniel.