The Lock command allows you to lock one or more of your opened files. When a file is locked, only the user who locked the file can submit changes to that file. Other users can open locked files in their workspaces, but they can't submit changelists that include locked files.
To lock one particular file,
Selecting Lock... from the pulldown menu on the Files tab brings up a form that lets you choose which of your opened files to lock.
If you have more than one pending changelist you can choose to lock the files associated with only one of them. To do this, check "Lock files in changelist", select the changelist you want, and click the "Lock" button. This locks the files in the selected changelist whether or not they are in the current path.
Alternatively, you can chose to restrict files to lock by path, or by selecting from a checklist. For more information, see Restricting Files Used in Commands. Click the "Lock" button after selecting the files you want to lock.
Only the files you already have opened in your workspace can be locked. (See also Unlocking Files.)