Hi Erik,
Those two views are not exactly comparable. The PLACEHOLDER_FEED_ITEM_REPORT will only return impressions where the particular feed item was displayed as an extension. The CAMPAIGN_PERFORMANCE_REPORT will return all impressions, regardless of whether those impressions displayed particular feed items.
In addition, the
ClickType field on both reports shows all click types
displayed for each impression. Therefore, if a single impression showed a headline and two sitelinks, you'll see that impression's data in two separate rows: one for "Headline" and another for "Sitelink".
The PLACEHOLDER_FEED_ITEM_REPORT has an additional field called
IsSelfAction that will be "true" for the
ClickType the user actually clicked, and "false" for the other
ClickTypes. This is unique to that report, as it's useful for seeing performance metrics for impressions where a specific feed item was not only displayed but was also clicked by the user.
Cheers,
Josh, Google Ads API Team

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