These signals are used to weight[1] the transmission characteristics of the system between the test generator and the output of the demodulator, where the system includes the microwave links, or TVROs as well as the TV transmitters and the transposers.
(Because analogue television is being phased out globally, VIT standards are considered superseded.)
In a composite video signal (CVS) there are two types of blanking: horizontal and vertical.
In a poorly tuned TV receiver the horizontal blanking can be seen at the right or left of the image and the vertical blanking can be seen at the top or bottom of the image.
In both systems, line numbers 17 and 18 are assigned for VIT signals in each field.