It’s important to understand how tracking works inside apps.

Let’s say you use a shopping app to browse for a blender. You look at a blender from Brand X, then close the app. Later, ads for that blender start showing up in other mobile apps, like Facebook and Instagram.

Here’s what happened: The shopping app hired an ad-tech company that embedded trackers inside the app. Those trackers looked at information on your device to pinpoint you. When you opened other apps working with the same ad-tech firm, those apps were able to identify you and serve you ads for Brand X’s blender.

Apple’s new privacy feature is intended to let you decide whether you want that to happen. Now, when you open some apps, you will be greeted with a pop-up window: “Allow [App Name] to track your activity across other companies’ apps and websites?” You can choose “Ask App Not to Track” or “Allow.”

When we select “Ask App Not to Track,” two things happen. The first is that Apple disables the app from using an Apple device identifier, a random string of letters and numbers assigned to our iPhones and that is used to track our activities across apps and websites. The second is that we communicate to the app developer that, broadly speaking, we don’t want our information to be tracked and shared with anyone in any way.

That seems easy enough. But No. 2 is where things also get slightly complicated.

Ad-tech companies already have many ways to follow us beyond Apple’s device identifier. For example, advertisers can use a method called fingerprinting. This involves looking at seemingly innocuous characteristics of your device — like the screen resolution, operating system version and model — and combining them to determine your identity and track you across different apps.

It’s difficult for Apple to block all tracking and fingerprinting happening on iPhones, privacy researchers said. That would require knowing about or predicting every new tracking method that an ad-tech firm comes up with.

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