Lines and Edges are synonymous in drawing 2d... The current Style determines how edges are displayed [see Styles > Model-Styles > Edit]. From your point of view there are two that need to be set/ticked. Edges and Profiles. The Edges setting is the number of pixels that the general edges show in, i.e. an edge having 2 faces. The Profile setting is the number of pixels an edge shows in - if its an edge with no faces at all or an edge with one face [i.e. on a perimeter of a face], it also ensures that any curved surfaces in 3d still display something even if the surface arcs away from the camera. Coplanar edges which form a continuous loop will automatically get a face. Additional edges can split a face into smaller facets if possible. The inner edges that have 2 faces will show in the Edge Style, the outermost ones or unfaced edges will be shown as Profiles... Sometimes edges do not split a face as expected. If you select the geometry [faces/edges] in question and right-click > context-menu > Intersect... they will usually split as much as they can. Obviously an edge that has a lone-vertex cannot split anything and will show as a 'Profile'...