The Codex classifies new discoveries per region. Cross over into a new region, and in the first system you visit, every single object is a "new dicovery" for your Codex. First Class III gas giant, first ice world, First Earth-like, etc.
The Codex only gives you the little message for the first ever such thing you find in each region. So, in your new region, if the first system you visited has two Class III gas giants, it will give you the message for the first one, but not the second, nor for any other Class III giants you might find in that new region (unless you happen to find one of the super-rare green ones, which are a separate Codex entry).
The whole "Reported" and "Confirmed" schtick seems a bit chaotic, as there is no definitve explanation I have found as to how to "Report" an object, and how it is then "Confirmed." It doesn't help that my Codex for the new region (Outer Orion Spur) now lists items that are "Confirmed" (I assume by me given the date listed) but not listed as "Reported."
You can't "report" stuff. "Reported" happens automatically when other people find and Confirm stuff; their finds then become visible as "Reported" to you. They become "Confirmed" for you only when you find and scan it (or one just like it) yourself.
Suppose, while you're travelling about the Outer Orion Spur, you happen to find a Green Class II gas giant. NO ONE has ever found one of these before in that Region, so in your Codex, it will appear as both Reported and Confirmed by you. This discovery gets immediately Reported by your ship's computer to every other commander in the galaxy, so that every other player in the game will now see Green Class II Giants as a new "Reported" item in the Outer Orion Spur, and your name emblazoned there as the Reporter. If I now want to make that Green Class II Giant "Confirmed" in my Codex, I need to fly there and scan it myself, but no-one else can steal the glory of being the Reporter of Green Class II Gas Giants from you.
Note that this Reporting happens immedately when you FSS the object in question, it does not wait until you sell the data. ANd the star system where you found it is public knowledge; everyone can now see it in their Codex. So if you do happen to find a really, really rare object with a brand new Codex entry for which you are the Reporter, rush back to base immediately and sell the data - otherwise, someone else is likely to fly out and steal the First Discovered By tag on that item. They can't steal your "Reported By" tag in the Codex, but they can steal your "First Discovered By" tag on the system map.