The manual says (on page 62):
You can move notes from one staff to another. This is useful if you edit several tracks.
1 . On the toolbar, open the Quantize Presets pop-up menu and select a quantize preset. This helps you when you move notes to specific time positions.
2 . On the extended toolbar, make sure L is deactivated.
3 . Select the notes on the same staff.
4 . Click one of the notes and drag them to the new system. The active staff rectangle indicates on which staff the dragged notes appear.
The notes are moved to the other staff.
(Cubase_Pro_Score_12_Score_Layout_and_Printing_en.pdf - WebHelp version)
I would replace the text in step 3 with one of the following versions:
→ Select multiple notes on a single staff.
→ From a single staff, select multiple notes.
→ Select multiple notes, but include notes from only one staff.