Countersink and Screw Size
A standard #10 screw has an outer thread diameter of 3/16". The #10 countersink you are referring to is a combined tool designed for applications where the screw is to pass completely through the material without digging the threads into the wood and have the head sit flush. If you wish to use a screw that is actually going to grip the material, such as a wood screw or self-tapping screw, do not buy the item. Instead you will need an appropriately sized #10 pilot drill (3/32" for soft wood, 7/64" for hardwood) and a separate countersink bit designed for your chosen screw size.



So, the answer depends on the application. If you intend for the screw to pass completely through the first work piece with nothing but the head securing it (no threads biting into the wood) then the combined #10 drill/countersink is great. If you intend for the screw to bite the material, you need a separate pilot drill and countersink bit.