What does the optical_flow_rad mean?

I just can not understand why pixel_flow_y/focal_length_px? why the unit of optical_flow_rad becomes radian?
//rad axis swapped to align x flow around y axis
accumulated_flow_x += pixel_flow_y / focal_length_px * 1.0f;
accumulated_flow_y += pixel_flow_x / focal_length_px * -1.0f;//rad

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if p is pixel in image,P is real world point,f is focal length,Z is distance from cam to P,
p/f = P/Z
r is rotation angle
r = atan(P/Z)
if r is small enough atan can be removed
r = p/f
if my answer is wrong Please correct it