What src attribute will do in amp-ad component?

Question

If i add the src attribute, which is available for amp-ad component as specified here, what will it do? Will it load the specified script before the amp-ad component comes into the visible area?

Code Sample:

<amp-ad type="taboola"
    src="https://cdn.taboola.com/libtrc/amp-demo/loader.js"
    width="400"
    height="300"
    layout="responsive"
    data-publisher="amp-demo"
    data-mode="thumbnails-a"
    data-placement="Ads Example"
    data-article="auto">
  </amp-ad>