Source Type (App Store Connect) is the App Store Connect Analytics dimension recording where a user came from before viewing or downloading an app, such as App Store Search, App Store Browse or App Referrer.
App Store Connect attributes each download to the source recorded when the user taps to download, and later sales, usage and subscriptions inherit it. The types are App Store Search (including search results ads), App Store Browse (for example the Today, Games and Apps tabs), App Referrer (links in other apps, including StoreKit product pages), Web Referrer (website links opened in Safari), App Clip, Institutional Purchase (Apple Business or School Manager) and Unavailable. Google Play Console splits traffic differently: its "Google Play search" source counts only searches for your app's name or brand, while category searches and autocomplete taps fall under "Google Play explore".
Source type is how an ASO team separates search growth from browse growth. A keyword win should show up as more App Store Search impressions and downloads; a feature or chart climb shows up in Browse. Because Apple Ads search results taps are folded into App Store Search, net out Apple Ads search results installs before crediting organic keyword work, or a new campaign will look like an ASO win. Filtering conversion rate by source helps too: in our experience search visitors convert differently from browse visitors, so a blended rate can hide which part of the listing needs work.
Example
A habit-tracker app's downloads rise after a subtitle update. Filtering by source type shows App Store Search up sharply while Browse is flat, but an Apple Ads campaign also launched that week, so the team nets out Apple Ads installs before deciding how much of the lift the new subtitle earned.