Give me more information on requirement 3.1.18 in the NIST SP 800-171 revision 2 document.
A mobile device is a computing device that has a small form factor such that it can easily be carried by a single individual; is designed to operate without a physical connection (e.g., wirelessly transmit or receive information); possesses local, non-removable or removable data storage; and
includes a self-contained power source. Mobile devices may also include voice communication capabilities, on-board sensors that allow the device to capture information, or built-in features for synchronizing local data with remote locations. Examples of mobile devices include smart phones, e-readers, and tablets.

Due to the large variety of mobile devices with different technical characteristics and capabilities, organizational restrictions may vary for the different types of devices. Usage restrictions and implementation guidance for mobile devices include: device identification and authentication;
configuration management; implementation of mandatory protective software (e.g., malicious code detection, firewall); scanning devices for malicious code; updating virus protection software;
scanning for critical software updates and patches; conducting primary operating system (and possibly other resident software) integrity checks; and disabling unnecessary hardware (e.g., wireless, infrared). The need to provide adequate security for mobile devices goes beyond this requirement. Many controls for mobile devices are reflected in other CUI security requirements.

[SP 800-124] provides guidance on mobile device security.
A mobile device is a computing device that has a small form factor such that it can easily be carried by a single individual; is designed to operate without a physical connection (e.g., wirelessly transmit or receive information); possesses local, non-removable or removable data storage; and
includes a self-contained power source. Mobile devices may also include voice communication capabilities, on-board sensors that allow the device to capture information, or built-in features for synchronizing local data with remote locations. Examples of mobile devices include smart phones, e-readers, and tablets.

Due to the large variety of mobile devices with different technical characteristics and capabilities, organizational restrictions may vary for the different types of devices. Usage restrictions and implementation guidance for mobile devices include: device identification and authentication;
configuration management; implementation of mandatory protective software (e.g., malicious code detection, firewall); scanning devices for malicious code; updating virus protection software;
scanning for critical software updates and patches; conducting primary operating system (and possibly other resident software) integrity checks; and disabling unnecessary hardware (e.g., wireless, infrared). The need to provide adequate security for mobile devices goes beyond this requirement. Many controls for mobile devices are reflected in other CUI security requirements.

[SP 800-124] provides guidance on mobile device security.