Check-in/Check-out
If a user wants to modify a file from a system library that file is "checked out" by the user for his/her exclusive use. While a file is checked out no one else can modify that file. When a file is checked out it is first copied onto the user's workspace and then locked from further modification in the library. When a file is checked back into the library the newly modified file is copied back to the library and then unlocked.