Difference between url and contentUrl?

For MediaObject, what's the difference between using the "url" and
contentUrl" property?

For example, if I want to declare the URL of an ImageObject can I use
either the "url" property ("URL of the item") or the "contentUrl" property
("Actual bytes of the media object, for example the image file or video
file")?

Looking at the MediaObject examples this is clearly okay:
<script type="application/ld+json">
{
  "@context": "http://schema.org",
  "@type": "ImageObject",
  "contentUrl": "http://www.airshock.com/images/acme-logo.png",
  "name": "Acme Logo"
}
</script>

But what about this - is it also correct, and how might it materially
differ from the code above?
<script type="application/ld+json">
{
  "@context": "http://schema.org",
  "@type": "ImageObject",
  "url": "http://www.airshock.com/images/acme-logo.png",
  "name": "Acme Logo"
}
</script>

Put another way, in what circumstance would or could the "url" of a
MediaObject be different than the "contentUrl" of a MediaObject?  And what
meaning exactly is "actual bytes" supposed to convey?

Thanks for any insights.