What's The difference between partial and full audience refresh?

On a single Audience, you have the option to run a full or partial refresh. These sync member data from Mailchimp to Salesforce.

A partial refresh will only update records recently modified in Mailchimp since the last sync. It is a member- and audience-specific version of primary sync (on Mailchimp Settings).

A full refresh will update all records in the associated Mailchimp audience. If you update a field mapping from Mailchimp to Salesforce, we recommend you run this full refresh or retrospectively apply the changes.