This allows not only general lines or cylinders, but customized "bonds" to be drawn.
The lines or cylinders are defined in terms of two points in fractional coordinates on the structure (crystal) axis system.
If the Radius is 0.0, lines will be drawn (if allowed in the particular type of display or output) - otherwise cylinders will be drawn.
If you have clicked with the mouse on two atoms (see Identifying (Picking) Atoms), you have the option, in the Generated Atom Data dialog, of defining a "bond" between the current atom and the last one. You will be referred to this dialog, and the "bond" will be kept in the list of lines/cylinders, not in the Bonds dialog in the Input1 menu.