Cannot ssh to server directly as root but non-root, then su to root is okay

On some servers, you can only login ROOT if you first login as a non-ROOT account, and then SU (NOT SUDO, BUT SU)

So, if you try to putty/ssh/.. directly as ROOT, with the password you know is correct, it doesn't work.

But then you login as anything non-ROOT, then SU to ROOT with the correct password, and it works.

In my mind, if you know the ROOT password, either login should work since you have the correct password. But, direct ROOT login can be disabled.

My question is: what is this setup named like, what is the context this is being setup in, or, how do you set that up ?