Any global variable defined in a config using __EXEC() will be stored in this namespace.
For example in the description.ext:
__EXEC(testVar = 1);
The variable "testVar" can be accessed during mission run time using getVariable
hint str (parsingNamespace getVariable ["testVar", 0]);
The above example would print 1.
This example however:
__EXEC(_testVar = 1);
Would print 0, because adding an underscore will make the variable local to the config in the same way that it would make it local to a script.
See PreProcessor Commands for more details on __EXEC()