Flexographic Printing on Custom Flexible Packaging. Important facts about how we print plastic bags.
How do you want the print to appear on your poly bag?
If you want the image(s) to be in the same place on every bag, you will want the print to be registered. The image will then appear in the same position from one bag to the next.
If you're more concerned that the bags just have a "label" than about the appearance and it's OK for the image(s) to be on the bag multiple times, moving vertically or horizontally and having partial images at the top and/or the bottom (where they could be cut off), then random would work.
Registered printing offers control over image position and random printing does not.
Another fact to think about when printing on clear bags is the appearance of the ink. Flexographic printing inks are not opaque. Black ink offers the most opacity, followed by white and then by other colors, depending upon how much black or white they contain in their formula. Printing one color, like black, on a clear bag is typically done when nothing more than a "label" is desired. A suffocation notice, printed with black ink on a clear bag, is an example of printing merely to have the information present on the bag. Appearance of the printed image is not a high priority for suffocation notices, part numbers and other general information most of the time. The main concern is that the necessary information is there.