As part of the Blocks and Layouts initiative for Drupal 8, we are decoupling "layouts" from themes. Contrib themes will still be able to provide their own special layouts designed to work best with the rest of the theme, though what we hope to enable for site builders is the ability to switch layouts without switching themes and to switch themes without switching layouts. Primarily what I mean by layout currently is what themes now do in page.tpl.php, though potentially, we'll be able to reuse layouts for arranging fields in a node. If you've used Panels before, this separation of layout from page.tpl.php is a concept already familiar to you.
When a site builder decides to change the site or a particular page or set of pages to use a different layout, blocks assigned to regions in the old layout need to be assigned to regions in the new layout. For region names that are the same in both layouts, this is easy. But to handle this smoothly when region names are not the same, we are working on introducing the concept of a region 'type'. See http://drupal.org/node/1816968 for details.
The help we're currently looking for on that issue is coming up with a set of type names to standardize on. Niche contrib layouts will be able to use additional custom types, but we are hoping that the most commonly used layouts stick to the standardized ones. Region names from some of the currently most popular themes are posted on http://drupal.org/node/1816968#comment-6641376, so we need a set of types that at least covers those use cases.
If you have feedback on this, please comment in that issue. Thanks.