UI Policies execute on the Client-side (browser).
Purpose: They are used to dynamically change the behavior of information on a form based on user interaction or data values.
Actions: They primarily control three attributes of a field:
Mandatory (Required)
Visible (Hidden/Shown)
Read-only (Editable/Locked)
Execution: Because they run in the browser, they provide immediate feedback to the user without needing to query the server or reload the page.
Reference:
ServiceNow Client-side scripting documentation classifies UI Policies (and Client Scripts) as logic that runs in the web browser.