When you update the current Help target, Doc-To-Help selectively incorporates any substantive changes you have made to the project since it was last built, and performs only those actions that are absolutely necessary. When you rebuild the current Help target, Doc-To-Help empties its output directory and recompiles the entire project, even if you haven't changed anything.
To update the current Help target:
On the Build menu, click Make Target.
Alternatively, you can click the Make Target button on the Project toolbar.
To rebuild the current Help target in its entirety:
On the Build menu, click Rebuild Target.