In PAM Unix/Linux Library modules, what does the .so stand for?

Question

Pluggable/Programmable Authentication Modules
Examples: pam_stack.so
pam_securetty.so
pam_nologin.so
pam_selinux.so
pam_console.so
pam_access.so
pam_time.so
what is the origin of the .so suffix? what did it or does it stand for?
And if you know, why is pam so complex that the pam.d configuration files call library modules which call other library modules which refer to (usually .conf) configuration files.
Is it related to how PAM was developed? Is there a reason for so many steps? And if you know a more appropriate forum to ask this question, please tell me that, also.

Answer 1

The ".so" suffix stands for shared object.
These are shared libraries just as Windows has DLLs.

The modules seems so complex simply because the whole stack has been broken into modules. You load only what you need but of course, you must fufil all dependencies.