Two separate apps from two different companies.
Highland is a screenplay conversion app from Quote Unquote Apps, which is John August's software company. Slugline is made by Clint Torres and Stu Maschwitz. Stu and John co-created Fountain, and the Slugline and Highland teams work together on supporting Fountain adoption in the industry.
Slugline's focus is on the writing experience, with real-time formatting, autocomplete, and a live Outline Navigator. Highland's focus is on converting among popular screenplay formats, including FDX and PDF.
Slugline has a Send to Highland feature to make FDX export easy if you have both apps.