[Mark McLeod]

The multi-tasking LabVIEW environment gets even more complicated when you recognize that vi's are operating in parallel and in fact operations within vi's are also operating in parallel.

There's nothing unique about Periodic Tasks.vi that's given it and all the tasks within it priority over Teleop.vi or Vision Processing.vi, etc.
In my example posted above the four Refnum Gets are all parallel tasks synchronized only by the While loop data tunnels.

Everything's clamoring for scheduled CPU time and somebody's going to have to wait sometime. It's non-deterministic in this framework. There are some low-level non-RT calls made too, but for our FRC purposes think of it as close-to-RT.

The timer alerts the task scheduler when it needs servicing again. In fact you can use a timer with zero time just to break a task out of the CPU and send it to the back of the scheduler queue.