This verb is fully documented in LOG_MESSAGE in the Microsoft SNA Server APPC Programmer's Guide.
The main advantage of using this verb to log messages is that a user-defined message file can be specified, containing messages specific to your application. This means that you are not restricted to the messages defined in the SNA Server message file.
Note that all messages logged using this verb are logged at level 12 in the error log file; it is not possible to log messages to the audit log file or to log at different severity levels.