The User-ID part of the above tracking

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Also, session integration should generally be turned on.
Session integration is a function that links the User ID to pages viewed before logging in if you log in during a session
. For example, let's say you view an Amazon product page while logged out, and then log in on the order screen. If session integration is turned off, the User ID will not be recognized when viewing the product page, but if president email lists session integration is turned on, the User ID will be recognized and linked even when viewing the product page.

Session Integration

Please note that this must be included in the tracking code of all pages to be analyzed .
In the Universal Analytics tracking code, insert it between the ga('create~ line and the ga(7'send~ line as shown below .

ga('create', 'UA-xxxxxxx-1', 'auto');
ga('set', '&uid', ' User-ID ');
ga('send', 'pageview');

code will actually contain the ID assigned to each user. In other words, you need to incorporate a program that will enter each user's User ID every time they log in. Please see the reference article below for details on the specific program.

Reference: GA Forum | Google Analytics Information Site | Tracking code to enable User ID function