It would not surprise me if many users are missing out on this very nifty feature. In the Nib preview window (the round window in the Brush Settings Docker or Ctrl + F8), under Nib Properties you can drag the Transparency slider to the left and right, adding transparency to a brush stroke. (It has basically the same effect as when you hold down the Alt-key while using the Paint tool (P), and dragging the on-screen slider to the left or right). This is very convenient if you, for example, like to add different transparencies to multiple brushstrokes on the same object. In contrast, the Opacity slider in the Object Manager Docker, adds transparency to everything on the specific object. For example, when you have created a custom brush of a human figure, or are using one of the default variants, imagine how you would paint and draw a crowd of people. Not all figures in the crowd will have the same density when standing together. To give contrast you add transparency, making them lighter or darker.

Tip provided by Stefan Lindblad, CorelDRAW Master, Illustrator, Graphic Designer & Artist.