Spline / Curve mode can be used when registering Line / Polygon to produce smooth line / polygon surfaces. The Spline tool uses an interpolation method that estimates values using a mathematical function that minimizes overall surface curvature, resulting in a smooth surface that passes exactly through the input points.
In the UTM Geo Map application, the Spline mode can be found in the Buffer/Overlay and Area/Distance modules.
The following video explains how to create Spline while registering Line / Polygons:
Read Next: Spline Settings