Manual:$wgLogTypes

Details
List of log types. The logging system has two levels: an event type, which describes the general category and can be viewed as a named subset of all logs; and an action, which is a specific kind of event that can exist in that log type.

Gotchas
If you are adding a log type, its name should be 32 bytes or less. If you choose a name longer than 32 bytes, the full name will be kept in the variable but only the first 32 bytes will go into the log table of the database, so that when you try to view records in your custom log, there will appear to be none.