What distinguishes a variable from a normal sweep are the options selected during the sweep operation. The first two control the orientation of the sketch plane as it moves along the sweep path, and the second two apply scale and twist factors to the sketch as it is swept. Swept solid command options. The choices are Natural, Spine, and Parallel. With Spine, the z-axis remains tangent to another curve referred to as a spine curve.
Parallel keeps it perpendicular to an existing plane.
This constrains the sketch plane to remain parallel to the existing plane. Choices are Natural, Guide Plane, and X-Axis Curve. X-axis Curve orients it so that when projected it follows the intersection of another curve. Again, we use the Natural option. The Scale option lets you apply a scale factor to the sketch as it moves along the sweep path.