UI Policy conditions are evaluated both on-load (if the On Load tick-box is selected on the Advanced view) and whenever an update is made to any field on a form in ServiceNow. When an update is made, the condition on the UI Policy is re-evaluated. If the condition returns true, then the UI Policy Actions run. If the conditions return false (and Reverse if false is enabled) then the opposite of the UI Policy Actions run.

Understanding that these events all happen within the browser (AKA the client) tells us that UI Policies are evaluated client-side. In the Advanced view of the UI Policy form, there is a Script section. In this section, if the Run Scripts tick-box is selected, two script input fields will be displayed: Execute if true, and Execute if false. Each script has a pre-defined default value, which acts as the scaffolding for the script:

function onCondition() { 
 
} 

Any code you write inside of that function will be executed client-side, and thus will have...