su root or login root

Hi,
I find there is some customized linux with application.
When I use login account root and type the password.
It is not allow to login.
But if I login with specified user and password.
Then I use command "su - " and type root passwd.
It allow you to switch to "root" account .
Or if i use "su - root" and type password .it is not allow to login again .
It is strange I cannot login if I specified to use "root" account .

How to make this setting ..
Please advice in detailed.

The correct way to login as root is to type:

su -

not need to add root after su -

Base on I know. su already mean "super user"
root Definition:
http://www.linfo.org/root.html