These snapshot is taken from Principle of Electromagnetics by Matthew N.O. Sadiku.
In this derivation I don't understand the highlighted step. In previous step we are defining a position vector R from element dl to point in space (x,y,z) which is equal to xax + yay + (z-z')az , this position vector is defined for a unique point.
However in the next step we are defining same position vector in cylindrical coordinate but we are defining only two cylindrical coordinate and \$ \phi \$ coordinate is missing. I know that Electric field intensity will be constant for all values of \$ \phi \$ for a given value of z. But I think position vector should be directed toward a unique point in space. Can anyone justify why we are not defining \$ \phi \$ in the highlighted step?