To better understand the difference, let’s go back to the Google Analytics definitions. According to Analytics Help, the source represents the origin of website traffic. It may be a search engine (e.g., Google) or domain (e.g., .com, .org, .edu). Medium signifies the general category of the source, for example, organic search (unpaid search) or web referral (referral).
The Source/Medium dimension combines the information about source and medium. For example, google/organic and fff.com/refferal represent Source/Medium.
Source and Medium are critical tools in Google Analytics as they tell website owners where users are coming from. There are valuable findings to find out the main traffic source and decide which deserves more attention. If, for example, referral traffic from Pinterest.com generates a large share of profit, it’s worth investing more in it.
What is not considered a “source” in Google Analytics by default? https://support.google.com/