З специіфкації:
"A catch clause that does not specify an exception_specifier is called a general catch clause.
Some programming languages may support exceptions that are not representable as an object derived from System.Exception, although such exceptions could never be generated by C# code. A general catch clause may be used to catch such exceptions. Thus, a general catch clause is semantically different from one that specifies the type System.Exception, in that the former may also catch exceptions from other languages."
Тобто general catch ловить всі виключення, в т.ч. й ті, що не наслідуються від System.Exception