https://www.cisco.com/c/en/us/td/docs/security/ise/2-7/admin_guide/b_ise_27_admin_guide/b_ISE_admin_27_deployment.html#ID185

Default Endpoint Identity Groups Created for Endpoints
Cisco ISE creates the following five endpoint identity groups by default: Blacklist, GuestEndpoints, Profiled, RegisteredDevices, and Unknown. In addition, it creates two more identity groups, such as Cisco-IP-Phone and Workstation, which are associated to the Profiled (parent) identity group. A parent group is the default identity group that exists in the system.
https://www.cisco.com/c/en/us/td/docs/security/ise/2-4/admin_guide/b_ise_admin_guide_24/b_ise_admin_guide_24_new_chapter_010101.html#ID1678

If you do not have a matching profiling policy, you can assign an unknown profiling policy. The endpoint is therefore profiled as Unknown. The endpoint that does not match any profile is grouped within the Unknown identity group. The endpoint profiled to the Unknown profile requires that you create a profile with an attribute or a set of attributes collected for that endpoint.
https://www.cisco.com/en/US/docs/security/ise/1.0/user_guide/ise10_man_identities.html