This is a really helpful update in GA4.
Previously, when importing user data, such as someone making an offline purchase, you could import it into GA4, but then you had to wait for that user to visit your website or app before accessing the imported data and including them in an audience.
Now, this information is immediately available and can be used to add users to an audience without waiting for their activity on your website or app.
The uploaded data is stored in user properties and is associated with each subsequent event triggered by identified users.
This opens up quite a wide range of possibilities for segmentation and remarketing.
If you work with GA4 to BigQuery exports, be sure to check out my SQL cheat sheet.
