Definition of 

App referrer

What's 
App referrer
: it is one of the possible values for
Source Type
. It means:
The users tapped a link in an app that brought them to your App Store product page. Includes apps using the StoreKit API to load your product page. Includes Apple apps, such as Messages, except Safari.

See other fields from

Analytics: Installations and Deletions

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App Store Connect

The (only) data hub made for apps