Captive screws and shoulder screws are two different types of screws.

A captive screw is a screw that is designed to not become completely detached from the component it is fastening, even when fully unscrewed. This is achieved by attaching the screw to the component through means such as a spring or a retention washer. Captive screws are often used in applications where regular maintenance or repair may be required and losing small parts like screws could cause issues.

On the other hand, a shoulder screw (also known as a shoulder bolt) has a shoulder between the head and the threaded portion of the screw. This shoulder provides a precise stopping point for other components that are being assembled onto the screw. The unthreaded section of the shoulder screw allows for a tighter fit between the screw and its mating hole, which can improve repeatability and accuracy in certain applications.

So while both captive screws and shoulder screws have unique features that make them useful in specific applications, they serve different purposes.