Reference:
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.)

Section: Policy Enforcement
Explanation/Reference: https://www.cisco.com/c/en/us/td/docs/ios-xml/ios/security/s1/sec-s1-xe-3se-3850-cr-book/sec-s1- xe-3se-3850-cr-book_chapter_01.html#wp3404908137

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