To simplify my GTD system I would describe it as being broken up into larger outcomes and next actions.
Larger outcomes are categorized as:
Next actions are categorized as:
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Larger outcomes are anything that requires more then one step or a categorization that is important to me in my life. For example:
Get my bike fixed would be a larger outcome categorized as project because it can be completed regularly quickly (over a week or so), and is made up of next actions - call bike shop, budget the money, take bike to bike shop, followup call, etc
Maintain Healthy Lifestyle is a larger outcome categorized as a responsiblity because in addition to requiring next actions it may require projects, habits, etc.
To make sure i am doing what I should be doing I verify that what I am doing fits into one of my larger outcomes.
When I set a next action it may come from a project or it may just come to mind. If something comes to mind I can decide if it is worthwhile by asking myself, does this fall into one of the larger outcomes that is important to me, or should I set a larger outcome that would add to my quality of life? If the answer is no, the item might not be worth doing. I can also follow the flip side of this and review my larger outcomes (during my weekly review) to verify that I have projects and next actions set for them, this makes sure I am working towards the things I have decided are important to me.