In HSS truss connection when checking sidewall local yielding In-plane and Out-plane to rectangular HSS, the Zb (Plastic section modulus) is taken the same for both in-plane & out-plane.
Zb is the plastic section modulus not Young's modulus. I do agree though that it is about the axis of bending (in the branch member), so if the branch member is not square the in-plane and out-of-plane values should be different.
Thank you Tank