Google Analytics - User Identification

About

Id in Google Analytics for a User

The user id is the named id (the id of your application)

You can specify a user id when creating a tracker.

ga('create', {
  trackingId: 'UA-XXXXX-Y',
  cookieDomain: 'auto',
  name: 'myTracker',
  userId: '12345'
});

from doc

The client id is an id generated if the user id is not specified.