In google analytics, client id and user id are two different features similar to behavior flow and user flow. Below is a difference explained:
A bit more explanation about the difference between client id and user id.
- Client id represents an unknown browser or device while user id only represents a unique user.
- Client id is randomly generated by universal analytics cookie _ga. For user id, you have to pull the login ids from your authentication system and send to GA as a user id.
- Client id is assigned to all website visitors while user id is only applied to login users(you can not track what a non-logged visitor did with user id).
- Client id reports are available in every reporting view (Audience >> user explorer) whereas user id reports you need to create new reporting view.
- Client id consists of a random number with time stamp separated by a dot. User id only consists of random alphanumeric characters. Time stamp means the time of the first visit.
- User id can exist across multiple devices which makes cross-device tracking possible in GA. Client id only exists on the device or browser it has been setup.
- The attribution of user engagement is possible with user id because it can exist on multiple devices while with client id create attribution issues.