User ID enables the analysis of groups of sessions, across devices, using a unique, persistent, and non-personally identifiable ID string representing a user. First, you have to make the Google Analytics account for featuring User ID and then execute your tracking code. There is the necessity to create a User ID to evaluate User ID data which you can find in your Google Analytics reports. After creating the User ID, you have to find a property level by clicking on Admin. After clicking, Admin have to select account and property column where tracking information is available. All the procedure should be followed step by step for successful result. There is an option of edit after completing all the steps to modify your selection.
While following the steps, click to ‘ON’ which indicates that you have already read and agree to the User ID Policy. After selecting the agree policy the feature of User ID is activated in your account. For using User ID, it is necessary to generate your unique ID and assign ID to any new user. You can also reassign ID to returning users as the data of IDs are sent to Google Analytics. This User ID is used for your website and mobile application both but you can’t merge both in one view of reporting. Date of web is only send to web property and the data of mobile is send to app property only.
Data of User ID can only be seen in a dedicated User ID view. For analyzing the data which is collected through the User ID in the tracking code you have to generate User ID views in Google Analytics account. There is an option to generate more User IDs views on User ID property anytime which follows the standard process of creating new view. Standard views is not converted into User ID and vice-versa.
You can use two different methods to implement User ID on internet including that you have to provide your persistent, unique and non-personally identifiable ID. This ID is offered by an authentication system. Another method includes that you should have a suitable user ID which is directly set on the tracker. When user signs in to your website then this ID is automatically sent to Google Analytics. Website which implements authentication system and PHP can be updated dynamically. With the updating of the website you will also set User ID.
Source : SEO Tech Experts