I am experiencing much confusion over relations in SW sketches. I have a profile sketched on the front plane, a circle. I have a spline on the right plane, arcing over the front. What I am trying to do is pierce the end of the spline located on the right plane, through the edge of the circle on the front plane. The cursor will sometimes inference the end of the line, sometimes it won't. Sometimes when I am able to get the desired points displayed in the relations window, the options are grayed out/disabled...but sometimes they're available. I don't understand.
What are the rules here?