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A: Yes, UltraCart captures the session ID out of the _ga_<container id> cookie on the browser and uses that value when passing Google Measurement events. This can be observed in the integration logs for the Google Measurement integration.
Q: Can I track a GA4 session across multiple domains like www.mydomain.com and secure.mydomain.com
A: Yes, make sure that you configure GA4 for cross domain functionality. Also we recommend deploying UltraCart Analytics to your external site so that UltraCart can observe the entire session end-to-end and collect necessary identifiers to drive the Google Measurement API.