In today's multi-screen era, users can access your site via computer, tablet and mobile phone, you know how many users both a computer user is also a mobile phone user? Just use your phone to access your site do? You know the different screen, multi-screen output user do?
All of this can be through cross-device reporting solution, to be applied in this report, you need to open the GA in the Userid function, Userid is to solve the problem of cross-device user unique identification, but also has the disadvantage that users only log on when it can be recognized, and now the APP is not included in reach across devices inside.
The principle: this is the use of the Userid, with the background required, the user at the time of registration, generates the corresponding User ID in the background, the User ID is a string of numbers, it can be a character, but if only, then this User ID or into the page as a Cookie, and then to read through GTM, can be converted into custom dimensions.
See the specific implementation process with the background data with the User ID data to get through the GA Chapter II of.
This realization look at cross-device tracking, in theory, GTM can also be achieved by APP. Then you select the "Audience" → GA in the "Cross-device reports" you can see in Figure 2-54 below:
Figure 2-54 Cross Device reports
Only in this report have enabled User-ID view is very intuitive, you can see across devices using the ratio of division.