The "Augmented" means that there were elements of the evaluation that exceeded the requirements for EAL5, specifically in the areas of flaw remediation and vulnerability testing.
STOP is a proprietary OS that emulates Linux as its API and ABI are the same. Thus many GNU/Linux applications can be copied and executed on the XTS receiving their security protection from the XTS without the need for recompilation or recoding. The XTS-400 and its predecessors have a 20 year history of being used (in the 3 letter agencies of the US government and by friendly foreign governments) as guards that allow security rules to be imposed on information flowing from one security domain to another. And as application hosts where connecting to networks of different classifications or sensitivities is required.