Fundamentals

Advertising Identifiers: What They Are, What They Do, and Their Limits

By Daniel Sardá · Published on October 21, 2026

6 min read1,201 words

Advertising identifiers can connect activity and measure campaigns without necessarily including a name, but they do not guarantee anonymity or establish a person's identity on their own.

An advertising identifier is a string of characters that makes it possible to recognize a user with relative consistency within a device or digital environment. On mobile devices, it is typically assigned by the operating system so that apps and advertising services can relate certain activity for purposes such as measuring campaigns, limiting repeated ads, or personalizing ads.

The identifier is not, by itself, someone's name or proof of their civil identity. It is closer to a technical label: it helps recognize the same environment, but it does not show who is holding the phone. Even so, it can be combined with activity, accounts, or other data to support inferences about interests and habits.

Key idea: An advertising identifier can link activity without directly revealing a name. That separation reduces direct identification, but it does not eliminate the possibility of profiling.

What it actually identifies

The word “identifier” can suggest that a code maps exactly to one person. In practice, it is useful to distinguish several levels:

• The device or its user: the system may assign an identifier to the user configured on a phone.
• The app: each service records its own activity and may use permitted signals to recognize repeat use.
• The account: an email address or signed-in profile used across several screens can connect experiences that a mobile identifier alone would not connect.
• The person: attributing all those signals to a specific individual requires additional information, and errors, shared devices, or accounts used by more than one person remain possible.

For that reason, it is more accurate to say that an advertising identifier recognizes a technical environment or the user profile on a device, rather than that it “uniquely identifies a person.” The distinction matters: it avoids overstating both the precision of the technology and its supposed anonymity.

Ad ID and IDFA: two familiar examples

On Android, the Advertising ID, or Ad ID, is an advertising-use identifier associated with the device user. Android’s documentation explains that the user can reset or delete it. If it is reset, the previous value is replaced with another; if it is deleted, apps receive a restricted signal instead of a usable value.

In Apple’s environment, the best-known equivalent is the Identifier for Advertisers, or IDFA. Since iOS and iPadOS 14.5, an app must request permission through the App Tracking Transparency framework to access the IDFA when it intends to track. That permission is part of a platform policy and should not automatically be confused with every legal requirement that may apply in a given country.

Ad ID and IDFA are not identical, nor do they represent every possible identifier. Their access conditions and controls depend on the platform. Limiting them also does not prevent an app from processing data needed for it to function.

What they are used for

Advertising identifiers allow a network or platform to recognize repeated activity without necessarily relying on a name. According to Google Ad Manager documentation, in mobile apps they can serve functions similar to those that cookies often perform on the web.

Common uses include:

• Frequency capping: preventing, for example, the same ad from appearing too many times over a period.
• Campaign measurement: estimating how many devices a campaign reached or how an audience responded.
• Attribution: connecting an install, registration, or purchase to an earlier campaign probabilistically or according to the platform’s rules.
• Ad personalization: selecting ads based on activity or inferred interests when permissions and settings allow it.
• Remarketing: showing ads again in connection with a previous interaction.

These functions can fund services or reduce repetitive advertising. That usefulness does not settle the privacy question on its own: to make a decision, a user needs to know which signal is used, for what purpose, and with whom it may be shared.

Useful distinction: Personalized advertising uses signals about activity or interests; contextual advertising is based on the content or situation at hand. Limiting an identifier can reduce the former without making the latter disappear.

It is not the same as a cookie, an account, or an IP address

Although they can take part in similar processes, these signals work differently:

• A mobile advertising identifier usually comes from the operating system and is designed for advertising uses in apps, subject to its controls.
• A cookie is information stored in a browser context and follows rules of its own concerning domains, browsers, and permissions. Some apps also display web content, so the two mechanisms can coexist.
• An account identifier belongs to a particular service. It can follow a user across devices when they sign in.
• An IP address identifies a network connection point at a given time. It can change, be shared, and provide approximate information, but it is not equivalent to an advertising identifier.
• Device fingerprinting is built by combining characteristics of a device or browser. It does not necessarily depend on a resettable code and raises different control questions.

From a legal perspective, there is likewise no universal answer to whether every advertising identifier is personal data. The classification depends on the information available, who handles it, the specific uses, and the applicable law. In practical terms, it is sensible to treat it as a signal that can link activity, rather than as harmless data simply because it does not contain a name.

What changes when you limit, reset, or delete it

Mobile systems offer controls, but each action has a specific scope. Resetting an identifier breaks direct continuity with its previous value for future uses; it does not show that a company has erased records it already retained. Deleting it or restricting access prevents or limits apps from obtaining that signal, but it does not switch off all advertising or all data processing.

An app may still show contextual ads, use account information, or measure results through aggregated methods. Withdrawing a permission also does not automatically erase data provided earlier.

Caution: Changing a technical signal alters what can be done from that point onward; it is not the same as deleting every profile, history, or connection created in the past.

This limitation does not make the controls useless. On the contrary, it makes it important to describe them precisely. A meaningful choice requires understandable options and predictable consequences, not promises of total invisibility.

Transparency for an informed decision

Advertising identifiers illustrate a recurring tension in the digital economy. They can measure campaigns, support advertising-funded services, and reduce repetition, but they also make it easier to connect behavior over time. Their practical legitimacy depends not only on their usefulness, but also on whether people can understand them and exercise real control.

That control begins with simple distinctions: a code is not a civil identity; pseudonymity is not anonymity; resetting is not deleting; and declining personalization does not mean ads will disappear. From there, control over personal data requires transparency about purposes, access, and retention.

The best question is not only whether an app “has” an identifier. It is also worth asking what activity it connects through that identifier, what it uses it for, how long it keeps the information, and what alternative it offers. Those answers turn a technical setting into a genuinely informed decision.