What does Vertical Control mean in GTD?
Also called Perspective and Vertical Focus, it refers to the appropriate alignment between your coordinated daily activities (Horizontal Control) and your higher perspective values.
It provides you with the necessary vision to identify solutions and define the right tasks, to make decisions and to face critical issues such as why this project is being done. Ultimately, it is the vertical control that defines your priorities.
The Natural Planing Model is a way to apply vertical focus to each of your projects. The Six-Level Model for Reviewing Your Own Work is another form of vertical focus, oriented to the hierarchy of levels in which your interests lie.