Transparently identify users with authentication realms - This option is available when one or more authentication realms are configured to support transparent identification using one of the following authentication servers:
Active Directory - Create an NTLM or Kerberos authentication realm and enable transparent user identification. In addition, you must deploy a separate Active Directory agent such as Cisco's Context Directory Agent. For more information, see Transparent User Identification with Active Directory.
LDAP - Create an LDAP authentication realm configured as an eDirectory, and enable transparent user identification. For more information, see Transparent User Identification with LDAP.
Details:
https://www.cisco.com/c/en/us/td/docs/security/wsa/wsa11-0/user_guide/b_WSA_UserGuide/b_WSA_UserGuide_chapter_01001.html#:~:text=Transparently%20identify%20users%20with%20authentication,User%20Identification%20with%20LDAP.