Parallel axis theorem proof in Kleppner and Kolenkow

In page 249 of Kleppner and Kolenkow, An introduction of Classical Mechanics, for the proof of Parallel axis theorem, he writes the vector connecting the axis(z-axis) to mass element is :

$$ rho_j= x_j vec{i} + y_j vec{k}$$

Now, if it is the perpendicular vector connecting the axis to a mass element then why is there $k$ component..?

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