There have been so many posts around the forums from people trying to make sense of various finds, and what the Codex is doing, so here is my simple and brief explanation of how it works...
First, each region has it's own full set of codex entries. When a commander, any commander discovers the first body of a specific type anywhere in a region, that particular body type goes Reported for all other commanders. When a commander personally scans a body that was showing as Reported, of that specific type anywhere in the region, it will show as Confirmed for that particular commander.
Now, some body types has additional variations such as Regular G type stars, Giant G type stars, and Super Giant G type stars. Those show up underneath the main category of G type stars.
Looking at the codex entries in the image below. First thing to notice is that some of the Rumored, Reported, and Confirmed icons are lit up, and others are not. The lit up Reported ones mean that there is a type, or sub-type that has been reported somewhere in the region, that I have not personally found.
First example is the O TYPE STAR. There is a lit up Reported icon with 3 next to it. This means there are three sub-types of O class star that someone else has discovered in the region, but that I have not personally discovered myself. This is indicated by the unlit Confirmed icon with a 0 next to it.
Take for another example, again, the G TYPE STAR. Notice how there is a 2 next to the exclamation point, and a 1 next to the check mark? This means that there are three total sub-types of G class stars. Two of them are still reported, and I have not personally found one of that type, and one is confirmed BY me because I have found one personally. If you look at the first image, there are two reported types and one confirmed type shown.
Look at the L TYPE STAR. The Reported is unlit, and has a 0, while the Confirmed is lit with a 1. This means that there is only one type of L class that has been discovered by any commander, and I have personally found one.
The same above applies to the geological, biological and planetary Codex subsections as well.
Hope this helps.
First, each region has it's own full set of codex entries. When a commander, any commander discovers the first body of a specific type anywhere in a region, that particular body type goes Reported for all other commanders. When a commander personally scans a body that was showing as Reported, of that specific type anywhere in the region, it will show as Confirmed for that particular commander.
Now, some body types has additional variations such as Regular G type stars, Giant G type stars, and Super Giant G type stars. Those show up underneath the main category of G type stars.

Looking at the codex entries in the image below. First thing to notice is that some of the Rumored, Reported, and Confirmed icons are lit up, and others are not. The lit up Reported ones mean that there is a type, or sub-type that has been reported somewhere in the region, that I have not personally found.

First example is the O TYPE STAR. There is a lit up Reported icon with 3 next to it. This means there are three sub-types of O class star that someone else has discovered in the region, but that I have not personally discovered myself. This is indicated by the unlit Confirmed icon with a 0 next to it.
Take for another example, again, the G TYPE STAR. Notice how there is a 2 next to the exclamation point, and a 1 next to the check mark? This means that there are three total sub-types of G class stars. Two of them are still reported, and I have not personally found one of that type, and one is confirmed BY me because I have found one personally. If you look at the first image, there are two reported types and one confirmed type shown.
Look at the L TYPE STAR. The Reported is unlit, and has a 0, while the Confirmed is lit with a 1. This means that there is only one type of L class that has been discovered by any commander, and I have personally found one.
The same above applies to the geological, biological and planetary Codex subsections as well.
Hope this helps.