Answer
A lock file is a temporary file that holds a file for use by a particular service. GMS employs two different types of lock files, tlk files and lck files.
tlk files are exclusive lock files, i.e. nothing else can access the file while a tlk file is present, this includes other GMS services. It is unlikely that you will ever see a tlk file on your system, as these are only present for a breif period of time when the file is first accessed.
lck files lock out other services but are not fully exclusive in that they allow access to all other GMS services, but not to anything else. These lck files are seen more commonly as they persist for as long a service is working with a file. For example, if post is busy sending a message out to another mail server a corresponding lck file will exist in the out directory for that message for the duration of its transmission.
Keywords:lock, tlk, lck,tck