GS,
If your checking rule considers 'replenishment lead time', then receipts outside of replenishment lead time are not relevant for ATP checking.
If all of that is true then learning to read CO09 is a project in itself
I started working with CO09 about 20 years ago. I will admit that my memory is not as sharp as it used to be, but I don't recall ever thinking it was particularly hard to learn. I guess if you are new to availability checking, CO09 could seem a bit obtuse. Some day you will actually face a REAL project. Only then will your really come to understand what complexity is all about.
Rather than continuing to ask questions, perhaps it would be best if you just got into your sandbox and 'played around' a bit. You can usually learn much more through personal experience than you could ever learn in any classroom (or an online forum too).
Best Regards & Good Luck,
DB49