Questions about unipotent matrices.

Question

I have a question about the notation in the paper. On page 8, the 5-th line from bottom, it is said that $u_J(t)$ denotes the upper triangular unipotent matrix with $t$ in position corresponding to the root $\alpha_J=\sum_{j \in J} \alpha_j$. I don't quite understand this. For example, let $u_J(t)$ in $GL_8$ and $J=\{3,4,6\}$. What is $u_J(t)$? Thank you very much.