Описание тега vsct
Visual Studio Command Table. A command table configuration file is a text file that describes the set of commands that a VSPackage contains.
Visual Studio Command Table
A command table configuration file is a text file that describes the set of commands that a VSPackage contains. The Visual Studio command table (VSCT) compiler compiles XML-based configuration files (.vsct files) into binary command table output (.cto) files. The resultant.cto files are the same as those that are created by using the command table (CTC) compiler to compile.ctc configuration files.
Source: msdn