Explanation
https://www.cisco.com/c/en/us/td/docs/security/ise/2-1/admin_guide/b_ise_admin_guide_21/b_ise_admin_g

Cisco ISE implements an ARP cache in the profiling service, so that you can reliably map the IP addresses and the MAC addresses of endpoints. For the ARP cache to function, you must enable either the DHCP probe or the RADIUS probe. The DHCP and RADIUS probes carry the IP addresses and the MAC addresses of endpoints in the payload data. The dhcp-requested address attribute in the DHCP probe and the Framed-IP-address attribute in the RADIUS probe carry the IP addresses of endpoints, along with their MAC addresses, which can be mapped and stored in the ARP cache.
https://www.cisco.com/c/en/us/td/docs/security/ise/2-
1/admin_guide/b_ise_admin_guide_21/b_ise_admin_guide_20_chapter_010100.html

Explanation
https://www.cisco.com/c/en/us/td/docs/security/ise/2-4/admin_guide/b_ise_admin_guide_24/b_ise_admin_guide Step 1 Choose Administration > System The Register button will be disabled initially. To enable this button, you must configure a Primary PAN.
Step 2
Check the check box next to the current node, and click
Step 3
Click Make Primary to configure your Primary PAN.
Step 4
Enter data on the General Settings tab.
Step 5
Click Save to save the node configuration.