Source is where a session came from (google, newsletter, chatgpt.com). Medium is how (cpc, organic, email, referral). Tools derive them from click IDs, UTM parameters and the referrer.
Why It Matters
Every channel report and attribution model starts here. If source and medium are wrong, everything built on them is wrong too.
In d8a.tech
Paid click IDs are checked first, then the referrer is classified (video, email, social, AI assistants, search engines, referrals), and UTM parameters override the result.