In this image, the spline on the left is being meshed into a renderable object, with xpSplineMesher.
If this is enabled, meshing will only take place at render time, not during editing.
This can speed up the viewport significantly.
Enabling this setting will make the mesh flat like a ribbon.
The direction of the mesh can be changed with the Plane Up parameter.
With Planar enabled, a 2D plane is being generated (rather than a 3D object).
Set as Free, by default, this sets the direction of the mesh.
The alternative settings are: World X, World Y, World Z, Local X, Local Y and Local Z.
This parameter affects what proportion of the spline will be meshed.