in Moodle 2.7!
The site administrator can define which log stores are available, and in which order, from Administration>Site administration>Plugins>Logging>Manage log stores.
Available stores are: Standard, Legacy and External database.
Note: A new Moodle site will have Standard logging enabled and Legacy logging disabled. A site which has been upgraded will have both enabled.
The default Standard log is very detailed and should meet most requirements. The Legacy log should be disabled unless you have old custom reports that need it. An External database log store also allows you to connect to an external log database.
From Site administration>Plugins>Reports>Manage reports, the administrator can see which log stores support which given report: