As of version 4.7, Drupal theme authors can define and implement any number of 'regions' for content to be rendered into. This improved regions system replaces the previous 'left' and 'right' sidebar placements and should enable greater flexibility with layout and design.
For themes not based on theme engines, available regions are defined in a
themename_regions() function in the
.theme file. Engine-based themes have their engine's regions (defined in a
.engine file) and may also implement their own regions.
Content is assigned to regions through the block system and also through
drupal_set_content() calls. For example,
drupal_set_content('left', 'Hello there') would assign the text "Hello there" to the left region (usually implemented as a left sidebar). Any regions defined in the current theme or its theme engine are available on the block configuration page.
The first region defined in a theme's
.theme file (or a theme engine's
.engine file) becomes that theme's "default" region, used for example as the default option for block placement in the block configuration pages.