Final answer:
In quick mask mode, to create a semi-transparent area for an anti-aliased or feathered effect, you should paint with grey.
Step-by-step explanation:
In quick mask mode, if you want to create a semi-transparent area for an anti-aliased or feathered effect, you should paint with grey. Painting with grey will create a partially transparent selection which can then be used to create smooth transitions between different areas of an image.
Using white will create a fully opaque selection, while using black will create a fully transparent selection. Changing the opacity of the quick mask itself will not directly affect the transparency of the selected area.