Explanation
The required control flag means that the PAM module is essential for the authentication process. If the module fails, the whole authentication fails. However, unlike the requisite control flag, the required control flag does not stop the execution of the remaining modules of the same type. This allows the user to receive feedback from all the modules, such as password expiration warnings or account lockout messages.
References:
Linux Professional Institute LPIC-2 Exam 202-450 Objectives, Topic 211: System Security, Objective
211.2: Configuring PAM, Weight: 5
PAM Control Flags, System Administration Guide: Security Services, Oracle
10.2. About PAM Configuration Files, System-Level Authentication Guide, Red Hat Enterprise Linux 7, Red Hat Customer Portal
4. The Linux-PAM configuration file, SCO Group