Files in /etc/pam.d/

[Rocky Hotas]

Hello!
The directory /etc/pam.d/ contains PAM policies for services. Some are pretty clear and unambiguous: /etc/pam.d/sshd is related to the ssh listening service. But some other are not. 
For example: in that directory, "login", "passwd" and "system" refer to very similar fields. So, I would like to ask:
- What exactly is the scope of *each* of them? Does exists a documentation about it?
- What is (if any) the hierarchy followed by them? Let's say that "system" (which contains system-wide login policy) and "sshd" have different statements: which one will prevail?
I have not found an answer to these questions on documentation (https://www.freebsd.org/doc/en_US.ISO8859-1/articles/pam/index.html). Moreover, nor "man pam.d" neither /etc/pam.d/README contain information about it.
Bye!

Rocky