What's a difference between "on" and "when" in the new angular defer block

Question

New Angular defer block comes with some new features.

We can use when element as well as on element. What's the difference between them?

@defer (when isReady) {
 <span appFoo>
  {{ 'foo' | upperCasePipe }}
 </span>
 }

vs

@defer (on interaction) {
  <span appFoo>
    {{ 'foo' | upperCasePipe }}
    </span>
} @placeholder {
  <button>Trigger loading!</button>
}

Answer 1

on is meant to be used with the built-in triggers: interaction, hover, immediate, idle, timer and viewport.

when works with component variables/methods.