It's called the extension knuckle arm in the parts diagram. It's fastened to the steering knuckle through the kingpin and the steering knuckle is fastened to lower control arm through the lower ball joint. The tie rod holds the steering knuckle in place but the kingpin allows the vertical and the steering knuckles to pivot independently. The FUCA prevents the upright from pivoting on a vertical axis. With a single heim joint on the FUCA it looks like the upright will pivot changing the caster. I had considered building a FUCA similar to this new design but I was going to use 2 heim joints side by side.
I ask because I had an aftermarket FUCA bend slightly which allowed the upright to pivot to the rear slightly and that caused a caster change that ate a tire.