At this time, only Code.org-hosted tutorials and content are assignable to sections. 

Many tutorials listed on Code.org/learn are hosted and managed by other organizations on their respective websites - we just promote them. Students can still access these other courses through code.org/learn, without being assigned that course from your dashboard. 

Any course listed on studio.code.org is assignable to sections. You can also use filtering options on code.org/learn to only show Code.org-hosted activities.