Spline MaskBasic Coord. Object
The Spline Mask is paramount to a Boole Object for splines (see your Cinema 4D reference manual). Use the Spline Mask to cut out, connect, etc. spline shapes. This works best when the splines are smooth and all in on the same plane.
This is a very good tool for quickly creating spline shapes. Spline Mask objects can be used everywhere where a normal spline can be used:
Furthermore, every (closed) spline shape can be displayed in polygon mode. A Spline Mask can be converted to a polygon object via the Make Editable or Current State to Object commands (Create Cap must be active).
With the introduction of Cinema 4D R17, a Spline Mask can be made a subordinate object of more than 2 splines. The target spline is always the first spline beneath the Spline Mask object:
For several modes - like the subtract modes - the target spline is important while for others (e.g., Union) the order of the subordinate splines is irrelevant.