Prompted by a question from
@Lollipop I've finally put together a comparative list of needle features from data sheets I was sent by Schmetz a year or two ago (procrastination champion here) and pulled out the details specifically for knit/stretch needles and serger/coverstitch.
There are several differences on shape of elements such as eyes and depth shank, both relate to thread control but the latter brings the stretch needles closer to the hook so the thread loop is less likely to slip off and cause a skipped stitch. It makes it very clear that the jersey needle is just a universal with a more rounded tip but nothing else.
The ELx needles have 2 grooves, a common feature for use in overlockers & coverstitch but the HAx1SP superstretch compensates for having a single groove by the fact that it's wider. This makes me sound a very sad individual but I have learned to live with it.