Standard knit vs. HD knit: what is the difference?
Both scarves are knitted the same way. The difference is the gauge, meaning the number of needles per inch on the knitting machine. An HD knit has roughly 60 stitches across the height of the scarf, versus 36 on a standard knit. Each "pixel" is smaller, so the curves of a crest, thin letters, a mascot or a date come out much closer to your artwork.
You can compare both in the online designer: switch between the two scarf types and the preview updates stitch by stitch.