Using Siebel Tools > Checking Out and Checking In Projects and Objects >
About the Check Out and Check In Process
Check Out and Check In is a source control mechanism for multiple developers working in the same repository. It allows you to check out objects from the server and download them to your local repository for editing. When you check out objects, they are locked on the server. This prevents other developers from checking them out and avoids conflicts that could result from multiple developers working on the same objects simultaneously. When you check objects back to the server, the lock is removed, and the objects are available for other developers to check out.
NOTE: You can lock objects directly on your local repository, without checking them out, but changes you implement cannot be checked in. See Locking Projects Directly in the Local Repository.