For some segmentation tools, pressing Ctrl while using them inverts the usual behavior (e.g. "Add" tool behaves like "Subtract" tool).
A suggestion to make this more visible would be to adapt the tool icon that is shown next to the cursor to the corresponding icon of the inverted tool. So when the "Add" tool is selected, pressing Ctrl while using should not only invert the behavior, but also show the icon of the "Subtract" tool next to the cursor.