Where does audience membership location data come from?

Mailchimp collects location data on its subscribers when they interact with Mailchimp forms and emails.

When someone subscribes using a Mailchimp‑hosted signup form, Mailchimp records their IP address and uses a geolocation service to estimate their location. 

Each time that contact opens or clicks your emails, the system re‑estimates their location using the IP on record. The more interactions, the greater the confidence in the result. 

Contacts imported into Mailchimp start with no location data until they engage via campaign opens/clicks.

When data is available we store this on the Mailchimp Membership record in Salesforce in the dedicated location field.  This field uses Salesforce Geolocation field type.

More on Mailchimp Geolocation