HA is the newest model that came out last year. Has an aluminum body (although the keyboard is plastic), 4GB of RAM, and Win 10.
I got this model and found it performed significantly worse than the T100 I got in 2013. Slow to boot up, slow for apps to open, etc. The speakers also sucked and the keyboard was made of very flimsy plastic. So I returned it and tried the T100 Chi, which is the premium model-- all metal, thinner and lighter, has a 1920x1200 screen, etc. The speakers STILL aren't as good as the original T100, but it's a nice little upgrade. I'm interested in upgrading it to Windows 10 but my experience with it on the T100HA really soured me.