Salesforce's one-line definition
Salesforce's parameter table names sfmc_id the Subscriber ID and describes it in one sentence: "This nonconfigurable parameter is required to create persistent user IDs between Engagement and Google Analytics." Engagement is Marketing Cloud Engagement, the name Salesforce's help docs use for the email product, and SFMC stands for Salesforce Marketing Cloud. The value ties a click to the subscriber the email went to, so that person keeps one ID across both systems.
The set it arrives with
sfmc_id rarely travels alone. Salesforce's example link carries it after six UTM tags and next to a second Salesforce value, sfmc_activityid. Salesforce fills most of them itself:
- utm_term: the link alias text from the content asset
- utm_content: the content asset ID
- utm_id: the journey ID
- sfmc_activityid: the activity inside that journey the link belongs to, so Google Analytics can report clicks for that step
Only utm_source, utm_medium and utm_campaign are configurable. The rest are required and set by Salesforce, sfmc_id included. Our utm_id and utm_content pages explain how Google Analytics reads those two.
When a Salesforce link has no sfmc_id
Email links carry every parameter set up in Parameter Manager unless link tracking is disabled for that email address. In SMS activities, Salesforce says Campaign Term, Campaign Content and Subscriber ID are not tracked, so do not expect sfmc_id on a text message link. The whole feature is switched on per business unit, through the GA4 Property Tracking setting in Parameter Management.
Before you forward the email
Deleting it leaves the destination untouched. For the sender, a click without sfmc_id cannot be joined to a persistent user ID, the one job Salesforce gives the parameter. Because it names a subscriber, delete it before you paste a Salesforce email link anywhere public, as you would Mailchimp's mc_eid. The UTM tags around it describe the email, not the person reading it.