File extension DIAGCAB used by Microsoft or OEM support to deliver customized installation and test update packages to Microsoft customers. To generate these files OEM vendors or ISVs use WTP or Windows Troubleshooting Platform.
Usually to open DIAGCAB file you will just need to double click on it. Sometimes DIAGCAB file is delivered together with DIAGPKG file so you can try double-clicking on DIAGPKG.
You can run a troubleshooting pack in the Windows PowerShell window. To get the package use the Get-TroubleshootingPack cmdlet. Then pass it to the Invoke-TroubleshootingPack cmdlet. If you run DIAGCAB package in the Windows PowerShell window, the interactions are translated to text prompts.
For more general information about files, file extensions and registry you can read one of the following articles:
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