If $T$ is the transpose we have for two vectors $(ab)^T=b^Ta^T$, but what is the meaning of $[x^T, y^T]^T$ in the following?
The concatenation $z = [x^T, y^T]^T$ of two column vectors $x \in \mathbb R ^n$, $y \in \mathbb R^m$ is also denoted $z = (x, y)$, so that $z \in \mathbb R^{n+m}$.