I feel like I’ve explained this dozens of times, but let’s try again.
In general, severe damage mechanics already existed in the old system, it was just named “destruction”. So basically if you e.g. teared off the wing of the plane in the old system, it was counted as a destruction and you received a full kill credit (with 100% kill score and 100% kill rewards). There is one major exception from this, which is an engine damage, that was considered a critical hit in the old system, now it’s severe damage.
Destroyed (severely damaged) plane in the old system looked like this:
Spoiler

If someone decided to finish this enemy, he would get an assist without a kill credit (67% kill score and 25% kill rewards).
In the new system the situation changed and right now this plane is considered “severely damaged”.
Severely damaged plane in the new system looks like this (pay attention to the nameplate color):
Spoiler

The person who severely damaged the plane in the new system only gets a partial kill credit (that counts for tasks and challenges, but doesn’t count for service record). That person also receive partial rewards (80% kill score and 80% kill rewards).
If someone else decide to finish the enemy in severe damage state he receives full kill credit (that counts for tasks, challenges and is also counted as a kill for service record). That person also receive partial rewards (40% kill score and 40% kill rewards).
So you can easily figure out in the old system both players could gain much more combined score and slightly more rewards from one enemy (167% kill score and 125% kill rewards) than in the new system (120% kill score and 120% kill rewards). Also the full kill credit changed from the person who severely damaged the plane (in the old system) to the person who finished it (in the new system).
Both systems are actually pretty similar. The old system was much easier to understand in my opinion, the new system is much more complicated, because it’s much harder to figure out who is severely damaged and who isn’t (the nameplate keeps the same color).
And I’m not even including critical hits problem in these calculations. Because in the new system the devs removed most critical hits. In the old system you would get a critical hit every time you caused a severe damage or finished the enemy. All these critical hits are gone in the new system (we are not sure if that’s a bug or not). The problem with this is, the critical hit is worth about half the assist (to be exact, one critical hit is worth 33% kill score and 10% kill rewards). These missing critical hits are actually something that I miss the most, because I could gain even 10 critical hits per battle before, now it’s at most 2-3. So you can do the math to see how much score and rewards I lose in each battle due to this critical hit change.
There are also many other issues this new system caused, by keeping “dead” planes alive for longer. You can read about them in this topic and in the previous severe damage topic. These problems are not easy to fix, so they will likely remain in the game.