Hi,
Thank you for contacting the Google Ads API support team.
Yes, your understanding is correct. As per the campaign resource document,
metrics.unique_users returns the number of unique users who saw your ad during the requested time period. This metric cannot be aggregated, and can only be requested for date ranges of 92 days or less. This metric is available for following campaign types - Display, Video, Discovery and App.
Also, metrics.average_impression_frequency_per_user returns the average number of times a unique user saw your ad during the requested time period. This metric cannot be aggregated, and can only be requested for date ranges of 92 days or less. This metric is available for following campaign types - Display, Video, Discovery and App.
I hope this clarifies your query. Feel free to reach out for any further concerns/ queries.
Thanks,
Google Ads API Team