The user code in an SC_MODULE is run either as an SC_THREAD or an SC_METHOD.
An SC_THREAD has a stack and is allowed to block.
An SC_METHOD is just an upcall from the event kernel and must not block.
Use SC_METHOD wherever possible, for efficiency.
Use SC_THREAD where important state must be retained in the program counter from one activation to the next or when asynchronous active behaviour is needed.