A user identifier or user ID is a piece of information that identifies a user and distinguishes them from other users. User identifiers apply to various legal and tech concepts.
According to GDPR, any information that can lead to the direct or indirect identification of an individual will be considered personal data. The types of identifiers include a name, an identification number, location data, or an online identifier (like an IP address or cookies). These identifiers refer to information about an individual’s tools, applications, or devices. If you collect personal data, you are subject to GDPR requirements.
User identifiers also play a significant role in analytics. Many analytics technologies can’t identify unique users if they use multiple devices across multiple sessions. Each time a user does this, a new user is counted. A unique user ID eliminates this issue, attributing all activity to one user in analytics reports.
More details about the user ID on the Piwik PRO help center and blog:
