TransactionScope and multi-threading

This is correct: the TransactionScope class uses the Transaction.Current property that stores its value in the field, which is marked with the ThreadStatic attribute.

The ThreadStatic attribute makes sure that the field value gets thread affinity, i.e. it has unique value in each thread. It's the recommended approach to share data within a thread. It's also known as Thread Local Storage (TLS).

The TransactionScope class just defines a transaction context in the current thread. It doesn't mean, however, that your code must accomplish all the job in that thread. I could imagine a complex calculation algorithm that uses multiple threads.

See MSDN:

You should also use the TransactionScope and DependentTransaction class for applications that require the use of the same transaction across multiple function calls or multiple thread calls.

So maybe look into DependentTransaction - in particular, there is a worker thread example, here.