The edX TNL team is planning to remove SemanticSectionDescriptor, a backwards compatibility feature that will map a <section> tag to import as a <sequential>. This feature is extremely confusing because it pre-dates the existence of Studio and its terminology, so it’s basically mapping a tag called section to Studio’s concept of a Subsection. It was used in some very early edX courses (~2012), but was never documented as far as I can tell. If you have a Studio-authored course, you shouldn’t have to worry about this at all.
Full details are in [DEPR-124] DRAFT: Remove SemanticSectionDescriptor mapping of "section" tag in edx-platform - JIRA
Target date for acceptance/removal is 2021-03-05.