To illustrate the algorithm for assigning ground-truth bounding boxes to anchor boxes, consider an IoU matrix $\mathbf{X}$. First, if the absolute maximum value in the matrix is $x_{23}$, we assign ground-truth bounding box $B_3$ to anchor box $A_2$, and then discard all elements in row $2$ and column $3$. Next, we search the remaining valid elements; if the largest is $x_{71}$, we assign $B_1$ to $A_7$ and discard row $7$ and column $1$. We repeat this process: finding $x_{54}$ assigns $B_4$ to $A_5$ (discarding row $5$, column $4$), and finding $x_{92}$ assigns $B_2$ to $A_9$ (discarding row $9$, column $2$). Once all ground-truth boxes are assigned, we traverse the remaining unassigned anchor boxes (e.g., $A_1, A_3, A_4, A_6, A_8$) and assign them to their highest-IoU ground-truth box only if that specific IoU exceeds a predefined threshold.