Explanation

Monitoring = provides advanced monitoring and troubleshooting tools that you can use to effectively manage your network and resources Policy Service = provides network access, posture, guest access, client provisioning, and profiling services.
This persona evaluates the policies and makes all the decisions.
Administration = manages all system-related configuration and configurations that relate to functionality such as authentication, authorization, auditing, and so on pxGrid = shares context-sensitive information from Cisco ISE to subscribers
https://www.cisco.com/c/en/us/td/docs/security/ise/1-4/admin_guide/b_ise_admin_guide_14/b_ise_admin_guide

Reference:
https://www.youtube.com/watch?v=0kuEZEo564s&ab_channel=CiscoISE-IdentityServicesEngine

Reference:
https://community.cisco.com/t5/network-access-control/cisco-ise-multi-auth-or-multi-host/m-p/3750907

Explanation
https://www.cisco.com/c/en/us/td/docs/security/ise/1-1-1/installation_guide/ise_install_guide/ise_deploy.html if your deployment has two nodes and you deregister the secondary node, both nodes in this primary-secondary pair are restarted. (The former primary and secondary nodes become standalone.)