How is marketing channel details populated in adobe analytics?

The "Marketing Channel Detail" value is populated by the processing rules for the Marketing Channel Report in the admin console. When configuring a rule, you set the criteria for defining how a hit will be bucketed into a channel, and in that same rule, you define what the "Marketing Channel Detail" value will be. This is not driven by classifications. I've included a screenshot here.

 You also have many options as to what the value is, such as a query parameter, referring domain/url, etc.

EDIT: Also including a reference to the documentation here: https://experienceleague.adobe.com/docs/analytics/admin/admin-tools/manage-report-suites/edit-report-suite/marketing-channels/c-rules.html?lang=en