The manual says (on page 64):
MIDI note durations/Display durations/Velocities
The note (1) durations, the display durations and the velocities in the overlay (2) are shown as rectangles, and the selected note duration or velocity (3) is shown in the color that you specified for Selected Events in the Preferences dialog (Scores–Colors for Additional Meanings page).
(Cubase_Pro_Score_12_Score_Layout_and_Printing_en.pdf - WebHelp version)
(1) Don’t you also refer to a note when you say display duration? If so, I think the word “note” should be replaced with something else (“real”, for example).
(2) I think that the order of the words should be changed, because the durations and velocities are not in the overlay. They are represented in the overlay (by rectangles).
(3) The way this paragraph goes on (after the comma) makes me feel that the text refers to something else than the previously mentioned “rectangles”.
This is better:
The real durations, the display durations and the velocities are shown in the overlay as rectangles, which—when selected—are shown in the color that has been specified in the Preferences dialog (Scores > Colors for Additional Meanings page > Selected Events).