Overhead structure types (also called tower types) are stored in a library that can be accessed when building line constructions. You can create new tower types or modified existing one within the Line Database.
The window below shows a tower type named Laurel. It has a 3-phase circuit and two ground wires. The conductors are shown from the perspective of an observer looking in the direction of the wires. The vertical bar for each conductor shows the sag between adjacent towers. When you select a circuit or ground wire in the diagram, the default parameters (wire type, position and sag) are displayed in the panel on the right.
The tower in this example has space for only one circuit. In general, a tower can have space for multiple circuits.