Let's assume that one of the threads is currently using the CPU. We'll examine the rules that the kernel uses to decide when to context-switch in this case. (Of course, this entire discussion really applies only to threads at the same priority—the instant that a higher-priority thread is ready to use the CPU it gets it; that's the whole point of having priorities in a realtime operating system.)
The two main scheduling policies (policies) that the microkernel understands are Round Robin (or just “RR”) and FIFO (First-In, First-Out). (There's also sporadic scheduling, but it's beyond the scope of this book; see “Sporadic scheduling” in the QNX Neutrino Microkernel chapter of the System Architecture guide.)