Sections labeled Interface are applicable only to interface development, and not to model development. If you're using Stella Architect, this means they only apply to work you do in the interface () window. If you're using Stella Designer, they apply to all work you do. If you're using Stella Professional, they don't apply to the work you're doing.
Not all sections relating to interface development are explicitly marked. If the broader material being covered is related only to interface work, the sections are typically not marked. It's only sections that appear in a context in which both model and interface are being described that are explicitly marked.