What does Predefined Work mean in GTD?
It is the second of the three workday activities composing the Threefold Model for Evaluating Daily Work. It refers to the work you do on solid basis, that is, when you have previously defined exactly what you have to do and you act upon those actions already clarified and organized in your lists. That means you are not improvising or getting driven by the most apparently urgent or appealing tasks.
This is what happens when you are working upon your GTD system, for example; and the shape your working style takes most of the time if you are really being productive.