HAx1 SP is regular Sharp Point needles, a.k.a. 130, 705, etc. EL-705 is the same needle system, but have slighly different grooves along the side to keep over lockers behaving (they tend to be speedy and go through multiple threads and loops). EL needles are slightly stronger and stiffer than (doesn't flex as easily) the basic 705 neeedles. There are sooooo many different needle types to select from in this system it's hard to get a complete overview. They are not the only enforced or sturdy 705 needle. I thought most overlockers were made to work with a basic 705 needle (at least those who take this needle system), but some can be a bit more fuzzy when it comes to some thread and fabric, I guess needle type too.