Q:

When the user of a shell is `root`, does running `su` on another user name ask for password of the other user? 

When the user of a shell isn't root, running su on another user name will ask for password of the other user. 
When the user of a shell is root, running su on another user name doesn't ask for password of the other user.  Is it correct? 
I searched in the manpage of su, which only says su would ask for the password of the other user, and doesn't say that it doesn't ask if the current user is root.  Am I missing something from the manpage? 
Thanks. 

A:

If /etc/pam.d/su contains the following:
auth            sufficient      pam_rootok.so

then root is unilaterally allowed to use su without further authentication.