Activity and script components can be reused as steps in an operation, while other components can also be reused in support of an operation.
Reusing a Component as an Operation Step
To reuse an existing component in an operation, drag the component name from the Components tab of the project pane to an operation on the design canvas:
Only components that are able to be reused in an operation are able to be dragged to the canvas. These include most activity types listed under Endpoints (excluding Salesforce activities), as well as Scripts. Transformations are not able to be reused as operation steps at this time.
Reusing Components in Support of an Operation
Components that are not able to be used as steps in an operation are not able to be dragged to the canvas from the project pane, but may be able to reused in other ways. Component reuse outside of an operation is covered in the documentation for each component: