PVC corner fittings that join either three or four pipes at a junction; four-way corners leave the top open for a third trellis layer, three-way corners close off the top with a finished look.
Jeremy notes the three-way corners on this tent do not allow a third layer, while the back tent has four-way corners in use on the first layer so he could add a third if needed. It is mostly a finish look preference.
Discussed in: Ep 022