There is only one file type that uses the .LOCKFILE extension:
Mac OS X PLIST Lock File
Lock file used by the application preferences component of Mac OS X 10.7 Lion; saved as an empty file alongside a corresponding .PLIST file (e.g., the lock file for com.myapp.plist is com.myapp.plist.lockfile; used for marking access to a program's PLIST file when an application is open.
If a LOCKFILE file is deleted, Mac OS X recreates it the next time the corresponding application is opened. For example, if you delete Apple Mail's com.apple.mail.plist.lockfile file, it will be recreated the next time Mail is opened.
Lock files can be found in the user's ~/Library/Preferences/ directory.
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Updated: June 7, 2015