/etc/xinetd.d/, /etc/xinetd.d

/etc/xinetd.d/, /etc/xinetd.d
Explanation:
The /etc/xinetd.d/ directory holds the files that configure the xinetd service when using several configuration files instead of an integrated configuration file. Each file in this directory corresponds to a specific service that is managed by xinetd, such as telnet, ftp, ssh, etc. The name of the file matches the name of the service. The files in this directory contain service-specific options that override or supplement the global options defined in the /etc/xinetd.conf file. The files are read only when the xinetd service is started, so any changes require a restart of the service. The /etc/xinetd.d/ directory allows for a modular and flexible configuration of the xinetd service, as well as easier management and maintenance of the individual service files. References:
* How to configure xinetd ? - Red Hat Customer Portal
* Understanding /etc/xinetd.d directory under Linux
* xinetd - Wikipedia

/etc/cron.allow
Explanation:
The /etc/cron.d/cron.allow file is a text file that contains the names of the users who are allowed to use the crontab command to create and manage their own cron jobs12. If this file exists, only the users listed in it can use the crontab command, and all other users are denied access12. If this file does not exist, the /etc/cron.d/cron.deny file is checked to see which users are not allowed to use the crontab command12. If neither file exists, only the root user can use the crontab command12.
To modify the /etc/cron.d/cron.allow file, the root user can use any text editor to add or remove the names of the users who will be allowed to use the crontab command1234. For example, to allow the user frank to use the crontab command, the root user can append the name frank to the /etc/cron.d/cron.allow file1234. The root user must always be included in this file, otherwise the superuser access to the crontab command will be denied4.
References: 1: Linux At, Batch, Atq, Atrm Command Help and Examples - Computer Hope 2: 107.2 Lesson 1
- Linux Professional Institute Certification Programs 3: /etc/cron.allow - Linux Bash Shell Scripting Tutorial Wiki - nixCraft 4: Controlling Access to crontab (System Administration Guide ... - Oracle