onLoad Client Scripts execute as soon as the system loads the form, and can be used to perform actions such as controlling the options that show in a drop-down field based on the values in the record loaded from the database.
Pro Tip: an onLoad Client Script can provide onChange-like functionality, as it allows you to access the DOM, and directly interact with form components using standard JavaScript APIs, or the ServiceNow g_form.getElement() method, at which point we can use the standard element.addEventListener() JavaScript API to monitor for keyboard, mouse, or other events.