What is the Site Studio hidden region?
The Site Studio hidden region is a useful holder for blocks that you want to render using the Site Studio block element.
If you want to place Drupal blocks in your layouts using the 'Block element', your block needs to first be placed within a region within Structure > Block layout. Once a Block has been placed, it becomes available to the Block element.
The Site Studio hidden region is a useful holder for blocks placed using the Block element as the region does not render to the front-end of your website.