Line is supposed to be for office type printing, so text and bar charts etc. Dot is supposed to be for printers. On the c500 the difference between screens was quite noticeable but on this machine the screens are very similar.
But in saying that you will find that some jobs will print better in one screen than for the other. For instance if you have a job that is a solid half-tone dot is quite noisy where as line usually does a pretty spectacular job. If you want to test this fill a page with a black at about 50% and print it on each screen you will notice a big difference.
Typically you wouldn't sway too much from DOT1/LINE1 but the DOT2/LINE2 have a coarser screen and do a good job of some text work, particularly reverse type. They are a bit more jaggy and prone to moire however.
