Yes, exactly. Sometimes people define relationships based on romantic involvement. Sometimes based on depth of emotional attachment. Sometimes based on sexual involvement. And, as you may have intuited, sometimes that people that you care about most are not the people that you're bumping uglies with.

I've heard triads referred to as either a V (A and B are both dating C, but not each other) or a delta (A, B, and C are each dating each other).

My own style of poly is a bit more loosely configured, and doesn't tend to include any polyhedrons, so mmmv.