If the application is not a multithreading application, the STACK64 statistics are for the one and only thread that executed, and the THREADSTACK64 statistics are all zero.
If the application is a multithreading application, and THREADSTACK64 was not suppressed, the STACK64 statistics are for the initial thread (IPT), and the THREADSTACK64 statistics are for the other threads. However, if THREADSTACK64 was suppressed, the STACK64 statistics are for all of the threads, initial and other.