To Mirror a Feature

Select the Mirror Feature tool.
Select the feature.
Enter a data point to define the mirror axis.
The feature is mirrored.
Reset to finish.

To mirror a feature, select the feature (1) and the axis location (2).

Result of mirror copy with Selected Feature Only turned: Off (left) — the feature, plus its features, is mirror copied. | On (right) — only the underlying slab is mirror copied, not its taper and blend features.