Explanation/Reference: https://www.emc.com/collateral/TechnicalDocument/docu88302.pdf (11)

Explanation
D@RE is included by default on Unity arrays and included in the Unity license file unless otherwise requested
during ordering. Therefore, all that is needed to activate D@RE is to install the D@RE enabled license file.
The license installation process is not specific to D@RE, and is required to be performed before using the
array. When logging into Unisphere for the first time, license installation appears as a step of the Unisphere
Configuration Wizard, prompting the administrator to install the license file obtained from EMC. If this
license file includes D@RE functionality, D@RE will be enabled once the license file is installed successfully
Note: Unity storage systems address sensitive data concerns through controller-based Data at Rest Encryption
(D@RE), which encrypts stored data as it is written to disk.
D@RE is a license controlled feature available by default on Unity storage systems, and, being designed to be
largely invisible to administrative end users, requires almost no effort to enable or manage. Enabling the
feature or backing up the encryption keys externally takes just seconds, and redundant keystore backups stored
on array ensure that user data is always as available as it is secure.

https://www.dellemc.com/en-us/documentation/unity-family/unity-p-vsa-install/02-vvx-c-ch- deploying-single-sp-unityvsa.htm

Explanation/Reference: