2.4.5. Creating PAM Modules
You can create or add new PAM modules at any time for use by PAM-aware applications.
For example, a developer might create a one-time-password creation method and write a PAM module to support it. PAM-aware programs can immediately use the new module and password method without being recompiled or otherwise modified.
This allows developers and system administrators to mix-and-match, as well as test, authentication methods for different programs without recompiling them.