googletag.defineSlot('/21849154601,12345/Ad.Plus-300x250', [300, 250], 'atf_unit')]/code]

the [i],12345[/i] defines the passback Ad Manager account. That much makes sense.

But beyond that? I have no idea. I [i]think[/i] that the ad unit on their end is named "Ad.Plus-300x250", so do I need to create the same ad unit in my Ad Manager?

I also came across a setting that claims to be important, but it doesn't appear to have had an impact for me. I modified this line:

[code].addService(googletag.pubads());

$URL = 'https://' . $_SERVER['HTTP_HOST'] . $_SERVER['REQUEST_URI'];

.addService(googletag.pubads()
 .set('page_url', '$URL')
);

<!-- in my case, this is already at the top so it's not necessary again -->
<script async src="https://securepubads.g.doubleclick.net/tag/js/gpt.js"></script>
<script>
// same here; this is defined at the top so it's not actually needed here again
window.googletag = window.googletag || {cmd: []};

// trigger this 500ms after the document is complete
var interval = setInterval(function() {
 if (document.readyState === 'complete') {
 clearInterval(interval);
 altFunc();
 }
}, 500);

function altFunc() {
 var i, unit;

 for (unit in ['atf_unit', 'btf_unit']) {
 if (
 (i = document.getElementById(unit)) &&
 i.style.display !== 'none' &&

 // my active ad units have at least 300px width set in CSS, so if it's
 // under 300px then ignore it
 i.clientWidth > 200 &&

 // if the height isn't greater than 50px then it must be empty
 i.clientHeight < 50) {

 // my Ad Manager ID
 var slot = '/12345/',
 size;

 googletag.cmd.push(function() {
  googletag.pubads().collapseEmptyDivs();

  // atf
  if (unit === 'atf_unit') {
  slot += '300x250_ATF_Adsense';
  size = '[[300, 600], [300, 250]]';
  }

  // btf
  if (unit === 'btf_unit') {
  slot += '300x250_BTF_Adsense';
  size = '[[300, 600], [300, 250]]';
  }

  googletag.defineSlot(slot, size, unit)
  .addService(googletag.pubads()

  // no idea if this helps, but it doesn't hurt
  .set('page_url', '{$_SERVER['HTTP_HOST']}{$_SERVER['REQUEST_URI']}')
  );

  googletag.pubads().enableSingleRequest();
  googletag.enableServices();

  googletag.display(unit);
  });
 }
 }
}