The KUID is the unique identifier of users in Salesforce Audience Studio. It is an abbreviation for Krux User ID.
In cookie-based environments the KUID is always an 8-character ID with uppercase letters, lowercase letters, numbers and special characters.
In cookie-less environments we use the Android AdID, the Apple IDFA or a proprietary hashed device ID, e.g. in Safari.
For some partners that require an exclusively lowercase ID, we additionally offer a lowercase-only 9-character ID called "user".
Please ask the Support team if you need to know how to access your respective user IDs on page.