Mask Mode / Trace Stroke
Minimum requirement: Qubicle Basic Edition
Mask Mode freezes selected voxels so they are not modified by the active tool. Only unselected voxels can be modified.
Here, with Mask Mode enabled, painting over the model with the Freehand tool only affects the unselected voxels:
Trace Stroke fills in the complete path drawn with a tool.
Without Trace Stroke enabled:
With Trace Stroke enabled: