Named

The most common named identity is an email address. Every other attribute can be used instead of an email address as long as it's unique. Usernames or User ID numbers are are also occasionally used as named identities.

Anonymous

A identifier for an anonymous user is normally generated (a surrogate key) as it does not require any central authority.

Whenever a person gives an identifiant such as an email address via a form, this information should be captured and the anonymous identity should be converted into a named identity.

Examples:

• Google Analytics - Client Id (cid) - Anonymous Id
• A daily changing random identifier using the visitor’s IP address and User Agent.
• More …see the dedicated article What is a Surrogate Primary key? known also as Substitute, Synthetic or Generated Key - Logical Data Modeling -

Fingerprint

An anonymous identity may be also targeted through their computer's browser. The browser is the identification. See Web - Browser Fingerprinting

Note that device-persistent identifier such as browser fingerprint is forbidden because these are considered personal data under GDPR.

Android Advertising ID

The android platform offers also a targeting service known as the Android Advertising ID (AAID)

On your android device, open Google Settings app > tap Ads > then find Your advertising ID.