Thickness Enter any number between 0 and 100. The usual range is between 0 and 20. The thicker the line, the more definitive the separation between two sections, but excessively thick lines lose their impact. This element is more easily set using percentages than pixels or points. If you use pixels, the line width will stay the same without regard to the viewer's screen size.
Percentage is always dependent on the viewer's screen size.
Alignment Not of much concern if the line is spanning the screen, but if you use smaller line widths, you can indicated the line's position on the page. Your web page will appear in the screen reflecting your changes. When you insert an image block, an American flag icon appears as the default image. You will also need to indicated the image's position on the web page. These alignment options are explained on the Image Edit Screen.