Meaning of words CTS
.net, c#, cts
Solution
That's the second option, it's a "type system" (the way you define classes, methods, attributes, ..) that is "common" to all languages on the .Net platform (C#, VB, F#, ...). Of course, I may remember wrong, but I'm pretty sure that the term 'CTS' refers explicitely to the way the CLR*) stores and uses the metadata that describe how everything is structured.
*) you've probably also seen the CLR term already. It's Common Lnguage Runtime. Its "common"-ness is liguistically structed in the same way. It is not a "common lanuage" runtime, but it's common "language runtime", it's an execution platform common to all.
However -- actually -- there is also some slight meaning in the "second way" you proposed. There are some types that are actually common, for example the most basic collections (ArrayList, Collection, HashSet, ...) or interfaces (IEnumerable, IList, ...). However they are called BCL, the Base Class Library.
Problem
As the English is not my native language I would like to know the exact semantic of words `Common Type System (CTS)`. Can they be interpretated as a system of common types or as a type system that is common for .NET Framework? That is whether the system is common or the types are common.:)