The tracking block
Opens are counted by a 1-pixel image that loads from mxAURA when the email is opened. Clicks are counted because link fields are rewritten to pass through mxAURA on the way to the real page. Both need a small block of HTML in the template.
Insert it once
In the mxAURA menu, choose Tracking (pixel + ID). It inserts one block that holds two tokens: the open pixel and a hidden tracking ID. Put it in a Rich Text or HTML module near the bottom of the email. Once is enough.
Do not put it in an image module or a link box. It is HTML, and in those places it shows up as text. The extension reminds you of this the first time; tick Don't show this again to stop the reminder.
What it looks like
A tiny image and a hidden span. Recipients see nothing. If a contact's tracking property is ever empty, the image falls back to a blank pixel on mxAURA, so nothing breaks.
When it is missing
mxAURA checks every template for the pixel. A template without it shows a Tracking warning in the app and in the panel, and open rate for its messages reads 0%. That is not poor performance. It is a missing block.
Related: Build a template mxAURA can fill, Which token goes where.