Source
First User Source, long-term attribution to the first point of contact.
Refers to the first user touchpoint — it’s based on the First User Source dimension. This shows where a user originally came from when they first engaged with your website or app. It’s tied to the User Acquisition: First User Source Report.
Session Source
Session-level attribution, where traffic came from for a specific visit/session.
Refers to the traffic source for an individual session regardless of where the user originally came from. It’s general information about that specific session and is tied to the Traffic Acquisition: Session Source Report.
Which View / Column to use when building a Dataset?
Source → User Acquisition: First User Source Report
Session Source → Traffic Acquisition: Session Source Report