Axis-level changes, exactly the shape and labels getPlanChanges() uses —
from is the earlier exam, to the later one.
The 2017 classification of each exam (derived, with the clinician's own per-axis overrides applied), so a stage or grade that moved is visible.
The periodontal measurements, where the change is numeric rather than a label: per-site probing depth and attachment level, and what moved by a clinically meaningful amount.
What compareExams answers: the same mouth, charted twice.