This policy setting controls whether users with the Microsoft Office Compatibility Pack for Word, Excel, and PowerPoint 2013 File Formats installed can open Office Open XML files saved with pre-release versions of Excel 2013. Excel Open XML files usually have the following extensions: .xlsx, .xlsm, .xltx, .xltm, .xlam.
If you enable this policy setting, users of the Compatibility Pack will not be able to open Office Open XML files created in pre-release versions of Excel 2013.
If you disable this policy setting, users with the Compatibility Pack installed can open files saved by some pre-release versions of Excel, but not by others, which can lead to inconsistent file opening functionality.
If you do not configure this policy setting, the behavior is the equivalent of setting the policy to Enabled.