Flat rate is common in broadband access to the Internet in the USA and many other countries. A charge tariff is a class of linear rate, different from the flat rate, where the user is charged by the uploads and downloads (data transfers).
MUI stands for Multilingual User Interface and .dll.mui files are simply libraries(Dlls) that contain resources dedicated to localization : for an application to support multiple languages,dlls containing specific resources must be available eg language resources(arabic,french,english,chinese....
More About DLL Files. The word dynamic in Dynamic Link Library is used because the data is only put to use in a program when the program actively calls for it instead of having the data always be available in memory. Lots of DLL files are available from Windows by default but third-party programs can install them too.