Querry regarding use of pam_succeed_if.so

Hi,
 
I am studying PAM.
 
I understood upto “auth sufficient pam_unix.so.”. If credentials are ok then authentication will be successful. And it wont load any other module.
But then why pam_succeed_if.so is there? I just did some work on it, I found it is only useful when pam_deny is disable.
 
auth        requisite     pam_succeed_if.so uid >= 500 quiet
 
 
Also it is used in “account” module type:
account     sufficient    pam_succeed_if.so uid < 500 quiet
 
 
Can you please explain me? I saw the documentation too, but didn’t understand the purpose of it.
 
Regards,
Neelesh Gurjar
I have no special talent. I am only passionately curious ! J
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