The velocimeters have their transmitter and receivers in physically different places. Sound pulses are transmitted from the central transmitter and received in the three (or four) receivers. What we are actually doing when we measure the Doppler shift in each transmitter/receiver pair is to measure the velocity of the particles along an imaginary line running between the transmitter and receiver axis. This is what we call the bi-static axis. When we select a nominal velocity range (referred to simply as the velocity range for the Vectrino Profiler) we are in fact setting the distance (or time lag) between the two pulses that constitute a pulse pair, where the nominal velocity defines the maximum velocity along the bi-static axis before phase ambiguities start to appear. Remember, the velocimeters measure phase shift between pulses in a pulse pair, where increasing velocity means increased phase shift, and phase shifts larger than +/- pi lead to spikes, or phase ambiguities. By decomposing the velocity along the bi-static axis onto the vertical and horizontal axes you get the corresponding maximum vertical and horizontal velocities. Because the sensitivity is at its highest in the direction of the transmitter and receiver, the smallest velocity range is in these two directions. Alternatively, think of the decomposed velocity as a triangle, where the bi-static axis (nominal velocity) is the hypotenuse, while the vertical velocity (along the transmitter axis) is the cathetus. The sensitivity in the horizontal direction, on the other hand, is much smaller. Here, the hypotenuse is the horizontal velocity range, while the cathetus is the bi-static/nominal velocity range.
The bottom line is that it is the vertical and horizontal velocity ranges that represent the maximum velocities that can be measured along these axes (listed in the rightmost panel of the Deployment Planning dialog). The “Nominal Velocity Range” is a term coined by Nortek as a simple way of describing the maximum unambiguous velocity the instrument would be able to measure. The Nominal Velocity Range controls how far apart in time the two pulses used to determine the Doppler shift are. The further apart they are, the higher the velocity that can be measured and the lower the noise. If they are transmitted closer together, the instrument can measure higher velocities, but will suffer from slightly higher noise. The rule of thumb is to set the velocity range as low as possible without risking the water velocities exceeding the horizontal or vertical velocity ranges.
When considering this you should keep in mind that in turbulent flows you very often can get much higher spurious velocities than the mean flow represents. This is why you may experience occasional spikes in the data when the mean flow starts to approach the maximum velocity, and this is even more the case near boundaries and physical structures, where the turbulence intensity tends to be higher.