NUL licensing provides specific users with an individual license. These named users will always be able to access web services because they will always have a license available. This contrasts with the CSL case, where users will be unable to access web services if all licenses are currently in use.
In order to use a NUL, the user's Windows username must be entered into the Users list on the Security Roles tab of the Administration Tool and be marked as Named Viewer or Designer. Please be aware that you will not be able to mark more users as named Viewers or Designers than you have NULs available. So, if your license key gives you 1 Designer NUL and 5 Viewer NULs then you will only be able to mark 1 user as a named Designer and 5 users as named Viewers. You can mark the same user as both a named Designer and a named Viewer.
Any web service that you want to license via NULs must be published so that it uses Windows authentication. Users will then be prompted to enter their Windows credentials to establish their identity. These credentials can be saved so that users only need to supply them once.