>Is this curved edge the initial seed?
Yes

>But it's neither horizontal nor vertical, why would the horizontal edges (and only some of them) be targeted from that seed?

Yes.I show how it works in plasticity if you can made this better its be cool
if we talk about the horizontal direction or the vertical i was wrong here
Better will be say edges that exist in the same plane or direction

>What is the targeting logic that only some of the edges with the same orientation are targeted and not all?

some edges in plasticity not selected for one use of this function if you can made algorith which can select all ring one side edges its be cool