Identify information system users, processes acting on behalf of users, or devices.
Bold Coast Security Guidance
Similar to the very first practice in Access Control, you must assign a unique username and password to users who will be accessing DoD information, CUI, etc. This also refers to unique service account names, host names, and infrastructure device identifiers.
The organizational policy must state that all users have a unique username and password, and all devices are uniquely identified. Your plan will describe how you manage these credentials, such as using Active Directory. You will need to review/audit the list of users and hosts on a scheduled basis for level four maturity.
DRAFT NIST SP 800-171 R2
Common device identifiers include media access control (MAC), Internet protocol (IP) addresses, or device-unique token identifiers. Management of individual identifiers is not applicable to shared system accounts. Typically, individual identifiers are the user names associated with the system accounts assigned to those individuals. Organizations may require unique identification of individuals in group accounts or for detailed accountability of individual activity. In addition, this requirement addresses individual identifiers that are not necessarily associated with system accounts. Organizational devices requiring identification may be defined by type, by device, or by a combination of type/device. NIST SP 800-63-3 provides guidance on digital identities.