Note: turning off multithreading only prevents multiple threads from being used for instruction simulation. It does not disable usage of additional threads for things like JIT compilation.
Disabling multithreading does not affect the threading mode of the simulation (see set-threading-mode). In particular, the thread domain partitioning and the virtual time synchronization mechanism is not affected.
The ability to turn off multithreading is primarily intended as way to help debug concurrency issues in models.