The Underbase strength controls how much white ink is used under black and dark areas. There are presets that are set up to give you the best color reproduction and typically, you want to use between Strong 100% white under black to 19 0% White under black.
With most DTG, you do need some white under black areas for the shirts to wash properly and not fade. The amount of white will depend on the pre-treatment and inks being used (but is normally quite high).
If you were to underbase the above image using Strong 100% white under black, 24 50% white under black, 19 0% white under black, and 9 Weak, the underbase would look like the images below.
The black areas represent the white ink and you can see left to right, with solid white under black, a 50% white tint under black areas, no white under black areas, and 9 weak where there is also no white under some darker colors.
These presents are set up to give you the best color reproduction and maintain good quality even after multiple washes.
If you want more control, there is a manual curve option available. It allows you to take complete control over how the white is used under the tonal areas.
Using the manual curve, you have complete control over how much white is used under the image based on the image saturation.
If you leave the curve as a linear line, with this setting, the white underbase on this image would look like this image below.
Setting the curve to:
With the manual curve, you have complete control over how much white is used, not just under black and dark areas, but can also control the amount of white used in the whole tonal range of the image.
If you are willing to sacrifice some color fidelity in some areas and potential issues when washing the garment, you can use this to reduce ink costs.
In this example this would be about a 10% saving in white ink.
As an example, using the below images, normally on a color shirt, the underbase strength would give you a white underbase such as the one below.
Whereas, using a manual curve such as the one below would print the white underbase as the one below.