Chapter 13 - Vector Geometry - 13.7 Cylindrical and Spherical Coordinates - Preliminary Questions - Page 699: 5

$\phi_0=\pi/2 $

In spherical coordinates, we have $$ \begin{aligned} &x=\rho \sin \phi \cos \theta\\ &y=\rho \sin \phi \sin \theta\\ &z=\rho \cos \phi \end{aligned} $$ Now, at $\phi_0=\pi/2 $; then $z=0$ and hence we get the xy-plane.