In base-aware mode, Rational Rhapsody DiffMerge reports differences with more details that describe what caused the differences. Rational Rhapsody DiffMerge is able to do so because it takes into account the third unit, the base unit. So each element (in the Left Value column) is matched not only with an element from another side (in the Right Value column) but also a base element (in the Base Value column).
All differences identified will be compared and put together to determine if they are trivial or non‑trivial differences. See Trivial versus non-trivial differences.