When a Polyline or multiple Polylines are selected, the SELECTION - TOOLS menu becomes available. The SELECTION - TOOLS menu contains tools that allow you to edit, change styling, annotate, or perform advanced functions on your selected Polyline(s). The menu dynamically adjusts the tools available depending on your item selection, with additional tools available for selecting more than one Polyline.

Individual Polyline Selection

Multiple Polyline Selection

Styling Group

LineAllows you to change the Polyline(s) color.
ThicknessAllows you to change the Polyline(s) thickness.

Annotate Group

NamePolyline(s) Descriptor & Number displayed over the Polyline(s) in the Viewport.
LabelAllows you to change the Polyline(s) Label color.
SizeAllows you to change the Polyline(s) Label size.

Edit Group

Edit VertexEnables you to edit the selected Polyline and its vertices. Displays the Edit Vertex Mode box where you can select editing options and change the coordinates (X,Y,Z) of the selected Polyline.

Transform Group

To TerrainSelected Polylines are draped to the underlying Elevation terrain by changing their Z value to the same elevation as the Z level of the terrain at their XY location. 
SplitEnables Split mode. Polylines can be split into separate Polyline items by left-clicking at your desired split location along a selected Polyline.

Result: Polyline
To BoundarySelected polylines are transformed into a Boundary item. Line segments between the gaps of the Polyline(s) are automatically filled in to complete the Boundary, and original Polyline items are removed.

Result: Boundary
DensifySelected Polyline(s) become more complex by adding vertices along the line that drape the Polyline(s) to the underlying terrain. Densify has 3 methods of adding vertices, Break Points, Regular, and Low-pass.

Break Points - adds vertices by elevation change along the Polyline.
Regular - adds vertices at specified distance intervals. 
Low-pass - adds vertices at the lowest elevations along the Polyline at specified intervals.

All vertices created always drape onto the underlying Elevation terrain.
SimplifySelected Polyline(s) or Boundary(s) become less complex by removing vertices along the line while preserving the geometric representation of the line or boundary. Simplify has 2 methods for removing vertices:

2D method - used for contour lines.
3D method - used for polylines and boundary project items.

Be sure to select the proper method when simplifying the line in order to keep the intended shape of your polyline/boundary on the terrain.
Merge PolylinesMerges two or more separate Polyline items into a single Polyline item. The Polylines must have endpoints that overlap with each other in order to merge.

Result: Polyline

Create Group

Offset LineCreates a parallel line to the selected Polyline. The delta Z Offset option allows you to shift your line vertically when creating the parallel line. A left-click in the Viewport determines the offset distance.

Result: Polyline(s)
Section LinesCreates a set of polylines along the selected polyline. The created Polylines are spaced perpendicularly along the selected polyline at regular designated intervals.

Result: Polyline(s)
Section ProfilesCreates a set of profile lines along the selected polyline. Created Profiles are spaced perpendicularly along the selected polyline at designated regular intervals.

Result: Profile(s)
To ProfileConverts any selected Polylines into Profile items. The selected Polylines are automatically disabled in the Project View and new Profile items are created in the same layer.

Result: Profile(s)  
Extract PointsCreates Point items at every vertex of any selected Polylines.

Result: Point(s)