Manual:$wgSharedTables

Details
This only includes the user table by default for backwards compatibility. However you may remove it so that you can enable extensions using the shared DB without having shared users. Or you can add in another table, such as interwiki to force MediaWiki to use the shared version of the table instead.

Be careful with sharing tables. The images, revisions, pages, and many of the other tables may have bad side effects if you share them.

In 1.16, the new table for storing user preferences was added - user_properties.

See Manual:Shared database for usage information.

Upgrading
As of Mediawiki 1.21,  must be temporarily cleared during upgrade. Otherwise, the shared tables are not touched at all (neither tables with, nor those with  ), which may lead to failed upgrade.

Example
If you wanted to share the ipblocks table, you would use: