“The complete junction catalogue for polyhedral scenes”
They call “Arrow”. I’ve seen it as W-shaped junction. But Arrow is better.
L-shaped junction would be the two edges of a plane. There’s no other indication of dimension , so it can be a plane.
Arrow junction (or W) would be where two planes meet. In an Arrow junction it looks like there are two planes. It is indicating THREE edges, two are on distinct connected planes with one SHARED edge.
A Y junction also shows THREE edges, but it also shows THREE PLANES, so there are THREE SHARED edges shown.