Like generic blocks, menu blocks could be configured to appear in the different regions of a page. Unlike generic blocks, the content of menu blocks is automatically generated to reflect the site’s menu structure. These blocks are often placed in the sidebar first region and provide an overview of the menu items available in that section of the site.
In WP2, menu level blocks were added by going to Structure > Blocks. This block is created by default with the site and lives in the “Disabled” section, until a site administrator chooses to configure it. It is called a Main Menu (levels 2+).
Migrating to WP3
There is no equivalent to the Main Menu (levels 2+) menu block in WP2. There is no way to display this information in WP3.