The essence of this document applies to both the "New" Facebook Pixel and the Conversion Pixels (old). This will hopefully help you understand better the connections (or lack thereof) between Firing, Tracking and Optimizing for pixels.
Here are the basics about pixel fires to get straight: There are 3 things to know about what pixels do: 1. Firing, 2. Tracking, 3. Optimizing.
Unless a pixel "fires" it can't do any of the other two. It's up to you to make sure when a pixel fires. That's why you need to put it in places where you want them to fire (on specific or all pages on your website, or in any links you control.) SOME pixels fire all the time and you sort all the pixel fires out with tracking and filters (see below,) others are fired only under certain conditions. You need to be aware and in control of when and where your pixels are firing.
Tracking: Tracking is a filtering feature that you can apply to a pixel that is firing. In some (rare) cases you would like to log EVERY pixel fire in the same place. Most of the time, you want to "track" only a select amount of pixel fire events so they can be "attributed" to a certain cause. For example, with your custom audience pixel (notice the singular here!) you FILTER the pixel fires that occur on a specific URL to build a specific CA of visitors of that page only.
Checkout or other conversion pixels (notice the plural here!) are filtered, too, so you can know which ad caused a specific pixel fire event. If you're interested in TRACKING a certain pixel fire event for a specific ad, you select that pixel in the Tracking section of the ad. You can track as many different pixels with a single ad as you want. But you have to be aware when and how the pixels are firing so you don't "over-track" (i.e. count the same event multiple times.) - and remember: Just because you select a pixel for tracking doesn't mean you'll see anything. You have to also make sure that the pixel fires in your funnel in the first place!
Optimizing based on a pixel is one step up from tracking. An ad set can only be optimized for a single event at a time. The pixel that is selected for optimization also HAS TO BE tracked by the ads in that ad set. The easiest way to think about optimization is that when you use your own pixels, you're allowing FB to get "personalized" feedback about what kind of people click on YOUR specific ads and take certain actions down YOUR funnel..
This goes beyond general statistics that FB has on their end about for example who engages a lot with posts, or who clicks a lot. These types of stats are used for all oCPM modes that are using "Facebook-side" data to pick who is seeing your ads next. When you use oCPM:Pixel you build a specific subset of FB user data that nobody else has. That's why this bidding mode is so powerful when you dial deeper into your own style and audiences.