It only occurs to me now, that there doesn't seem to be a generic back section, section.

The Annotation guidelines for the 'segmentation' model do not mention any back section. The existing training data wraps elements in a back section, probably to keep the general TEI structure.

It does support the following specific back section elements:

• listBibl
• annex
• acknowledgment

Out of those, acknowledgment is probably a sort of general back section (section).
But there could be others, e.g. relating to:

• funding
• competing interests
• author contributions

It would be good to be able to just extract general back sections (with title and paragraph(s)).
(Then acknowledgment could just be a special case of that)