Here is an impossible task for you: try to explain the difference between Level 1 / 10,000 ft projects and Level 3 /30,000 ft objectives in such a way that even I can understand it.
Is there a difference other than a) the timeframe of either longer or shorter than a year and b) Level 3 items don't have a Next Action item somewhere?
What is the purpose of maintaining a list for the Level 3 items?
As you go up the levels, the degree of abstraction is greater. So it's not immediately obvious how these two levels are different. It's subtle, and often doesn't become necessary until you've had more experience with GTD and there is more content in your system.
The reason the various levels become necessary is that as you start to capture and track all the outcomes you're committed to, your list could get pretty long and not all the items require the same cadence and type of focus.
It does basically come down to the timeframe and the degree to which they are immediately actionable.
For example, I am in grad school.
I have a project to complete the course I'm currently in (level 1)
Projects for the bigger assignments (also level 1)
all with various next actions (ground)
Completing the degree is not exactly a project I can immediately take action on, but it is an outcome I am committed to (Level 3) which would be distracting to review each week or more.
The goal of the degree is related to a larger vision of what I want to be doing professionally (level 4/5ish)
These are all tied into my Professional area of focus (Level 2)
When I first started with GTD I probably would have put most of those outcomes on my project list.
Eventually, it becomes apparent that some of those outcomes don't need to be reviewed every week (the level 3 stuff), and some aren't even outcomes (level 2).
That's when it becomes helpful to separate them. It provides the ability to focus on a different perspective of your commitments.
All of that being said, GTD isn't about forcing you to have these levels; it just gives you a framework to organize what's on your mind. Different people will have varying degrees of material for levels 3 through 5, if at all (as shown in this forum).