Select an editable spline. > Modify panel > Editable spline (not a sub-object level) selected in the modifier stack
Select an editable spline. > Right-click the spline > Tools 1 (upper-left) quadrant of the quad menu > Sub-objects > Top-level
The functions available at the editable spline object level (that is, when no sub-object level is active) are also available at all sub-object level, and work exactly the same at each level.
Interface
Mesh Settings, Interpolation and Selection rollouts
See Editable Spline for information on the Mesh Settings, Interpolation and Selection rollout settings.
Create Line: Adds more splines to the selected spline. These lines are separate spline sub-objects; create them in the same way as the line spline. To exit line creation, right-click or click to turn off Create Line.
PolyConnect: Turn this off if you don't want the new line connected to the endpoint of an existing line. When PolyConnect is on, when creating lines, if the initial mouse click is over an existing endpoint in the shape, the newly created line is automatically attached to that endpoint. This can be an annoyance, especially when creating spline networks for use in Surface Tools.
Attach: Lets you attach another spline in the scene to the selected spline. Click the object you want to attach to the currently selected spline object. The object you're attaching to must also be a spline.
Unattached splines (left) and attached splines (right)
When you attach an object, the materials of the two objects are combined in the following way:
If the object being attached does not have a material assigned, it inherits the material of the object it is being attached to.
Likewise if the object you're attaching to doesn't have a material, it inherits the material of the object being attached.
If both objects have materials, the resulting new material is a multi/sub-object material that encompasses the input materials. A dialog appears offering three methods of combining the objects' materials and material IDs. For more information, see Attach Options Dialog.
Attached shapes lose their identity as individual shapes, with the following results:
The attached shape loses all access to its creation parameters. For example, once you attach a circle to a square you cannot go back and change the radius parameter of the circle.
The modifier stack of the attached shape is collapsed. Any edits, modifiers, and animation applied to the attached shape are frozen at the current frame.
Reorient: When on, rotates the attached spline so that its creation local coordinate system is aligned with the creation local coordinate system of the selected spline.
Attach Mult.: Click this button to display the Attach Multiple dialog, which contains a list of all other shapes in the scene. Select the shapes you want to attach to the current editable spline, then click OK.
Insert: Inserts one or more vertices, creating additional segments. Click anywhere in a segment to insert a vertex and attach the vertex to the mouse. Optionally move the mouse and then click to place the new vertex. Continue moving the mouse and clicking to add vertices. A single click inserts a corner vertex, while a drag creates a Bezier (smooth) vertex.
Right-click to complete the operation and release the mouse. At this point, you're still in Insert mode, and can begin inserting vertices in a different segment. Otherwise, right-click again or click Insert to exit Insert mode.