The NU (Named User) subscription in Cisco's Flex Plan is based on the number of named users regardless of their usage. In this model, each user is given a license to use the Webex services, and the cost is fixed per user, making it ideal for organizations that have predictable usage patterns. This allows organizations to provide access to all users under a simple, per-user licensing model without considering actual service usage.
Cisco Reference
Details on the NU subscription model can be found in Cisco's Flex Plan documentation, which outlines how the named user licensing works and its benefits for predictable costs and user management.