Facebook's Aggregated Event Measurement is a protocol that allows for measuring web and app events from people using iOS 14.5 or later devices. Aggregated Event Measurement limits domains and mobile apps to eight conversion events that can be configured and prioritized for Aggregated Event Measurement reporting. At this time, only prioritized events can use Aggregated Event Measurement data to help improve conversion optimization.

Aggregated Event Measurement protocol needs you to:
Verify your domain on the Business manager
Configure and prioritize your top eight events on your domain
Set up Conversions API for Pixel

How Aggregated Event Measurement works with web events:

Your eight conversion events can come from one connection method (the Facebook pixel or Conversions API) or multiple connection methods. For example, if a business called Feroldi's has three pixels – let's say Pixel A, Pixel B and Pixel C – then Feroldi's will need to choose which eight events they want to include in their event configuration between the three pixels. If they choose Pixel A's purchase event, Pixel B's purchase event and Pixel C's purchase event, this would count as three of the eight events. After they choose their events, they would need to prioritise them.

In general, you'll follow these steps to configure web events for iOS 14.5 and later ad optimisation:

Verify your domains.
Domain verification establishes which Business Manager account has the authority to configure and prioritise the eight conversion events for a given domain. Please visit the link to see how to Verify your domain.

Configure and prioritise your top eight events on your domain.
You can choose up to eight standard events or custom conversions. Page view events are not available at this time. Visit Events Manager to access the web event configurations tool. Learn more about Event configuration.

Set up the Conversions API for Pixel to optimize conversion tracking
Conversion API comes in when the pixel is blocked by the browser. This is a crucial step to making data-backed ads decisions. You won’t want to make decisions on an eclipsed view of data right?
Please follow these documents for API setup and testing:
Conversion API setup steps
Server testing with Conversion API

You can also find more official information from Facebook about Aggregated Event Measurement in here.
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