Set Cut & Fill
In 
Inputs, select an input.⚓
Click Keying and Cut & Fill mode.
Select the cut content from the dropdown list.
The cut content must have the same format and capacity as the fill input and come from the same input card.
If needed, use the Curve setting to adjust the transparency level of the Cut & Fill.
If needed, enable the Black and White or the Color mask to ease the setting operations.