The Overlay (or Push-Down) feature of a Testing Platform Group (or its constituent Platform) is an extremely powerful feature that makes you very efficient in  making changes to any of the Testing Platforms within a (Project-level) Group, and pushing down the change(s) to Testing Contexts of Multiple Run Definitions and Scenarios so that you don't have to manually go in and make the edits to Testing Contexts of Multiple Run Definitions and Scenarios.

Let's assume that your automated test coverage involves certification of your application against the top 5% of platforms (browsers, operating system, and device combos) that your end customers use to access your apps. If new browser/O.S/Device combinations appear in your top 5%, you can very easily go in and make changes to the Testing Platforms within a project-level Group and 'push' down that change in one click to all linked Run Definitions and Scenarios, gaining significant savings in manual effort.  

The 'Overlay' (or Push-down) feature is available at the Testing Platform Group level as well as at the level of the Testing Platforms within a Group and below is the icon of Overlay:

Overlay/Push Testing Platform Group:

When you have made changes to more than one Testing Platform(s) within a Testing Platform Group, and wish to have the changes you have done to all those Platforms within that Group applied automatically to all linked Testing Contexts of one or more Run Definitions or Scenarios that were created using those Platforms within the Group, you can click on the 'Overlay/Push' icon for the appropriate Group shown on the 'Manage Testing Platform Groups' screen.

When prompted for confirmation, select 'YES' in the confirmation dialog for the changes to be overlaid on all related Testing Contexts of one or more Run Definitions and/or Scenarios.
  

Overlay/Push Testing Platform of a Group:  

When you have made changes to a single Testing Platform within a Testing Platform Group, and wish to have the changes you have done to that single Platform within that Group applied automatically to all linked Testing Contexts of one or more Run Definitions or Scenarios that were created using those Platform within the Group, you can click on the 'Overlay/Push' icon for the appropriate Platform within the Group shown on the 'Manage Testing Platforms for a Group' screen.

When prompted for confirmation, select 'YES' in the confirmation dialog for the changes to be overlaid on all related Testing Contexts of one or more Run Definitions and/or Scenarios.  

 
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