A User ID is a unique identifier that an advertiser assigns to recognize a specific user on their website or app. Instead of relying only on cookies or devices, a User ID helps track the same person across multiple sessions or devices when they are logged in.
User IDs are commonly used in analytics and conversion tracking to better understand customer behavior, such as how users move between devices before completing a conversion.
By using a consistent identifier, advertisers can get more accurate insights into user journeys, returning visitors, and overall performance. It’s important that User IDs are implemented in a privacy-compliant way and do not contain personally identifiable information such as email addresses or names.