Adding Drupal regions to your Master template
Drupal uses the concept of regions to divide up a page. The regions are used to house different parts of the page. For example, The Content region holds the page content, the header region usually holds the navigation and the footer region holds the footer content.
Drupal uses regions because in a standard Drupal website, site builders and editors are able to place blocks and fields within them. This is not necessary when using Acquia Cohesion as you are able to place blocks and other content using the Layout builder. The exception to this is the Content region. This must be added to your Master template as it's required to render your Content templates and Views templates within your Master template.
The content region
The content region is where your Content templates are rendered.
You must add the content region to the master template to render page content.
Using other regions
You can choose to add other regions to your Master template. This is optional but you may require Drupal regions for your specific website. The regions available to you are provided by the Cohesion minimal theme YAML file. This provides the same regions provided by Drupal's default theme Bartik. If you have access to this file you can add more regions if you require and they will become available to the region element.