Better mktoHTML variable explanation?

I'm reading up on a couple of the email variables, and I'm not understanding how to use the mktoHTML variable. I get the instructions, but it seems to be missing a crucial piece of information, or I'm being dense:

The Marketo explanation shows the metatag, and explains that it's boolean:

<meta class="mktoHTML" id="trackingPixel" mktoName="Add Tracking Pixel">

It shows the token that I'd place somewhere on the template:

${trackingPixel}

So far, that all makes sense. What I'm not understanding is... where do I put the actual html that one chooses whether to place.

So, in the example they give, where is the actual tracking pixel code, i.e. <img src="x.com/image.jpg"> etc. etc.