Hi JesusIsLord ! This is incredibly messy and complicated, but I’ll try and break it down for you as best I can.
First thing you have to know is that unless you have defensive units posted outside your bunker, it doesn’t do anything. It only improves the defensive performance of your deployed defensive units - it doesn’t protect your Sector by itself. If this is what you’re looking for, you’re better off investing in the Elite Sector Security units (the Robots with the “More” icon on them). These function as active defense units, and will chip away at attackers without costing you anything after the purchase price while also contributing to your SDB.
Secondly, there is some variation in how the battle mechanics work depending on how the attacker has broken down their forces. If the attack on a Sector is made by only one type of unit class (let’s say Infantry), it’s easy – each attack/defense point is applied directly against the relative unit after being run through all the artifact, upgrade, special bonus metrics.
If the attack on the Sector is mixed, (a player uses more than one kind of unit class) it’s broken down by unit category. If 20% of the attacking force is composed of Infantry, those total offensive points will go up against 20% of the total Defensive Infantry points of the defending force. If you dig deeper it gets more arcane, especially when you start adding all the other modifiers into the mix. We tried putting this all into the guide several times, but explaining how this works out in every situation without a chalkboard is pretty tough. If you do before-and-after comparisons, you’ll see the difference.
The simplest way to explain it is that you get a +1% boost to your total defense point calculation for every 100 SDB. So - if the total number of the defense points of all your defensive units is 30,000 and you have 2,000 SDB, that comes out to +20% overall, and your total aggregate defense points would be 36,000 during an actual battle.
How this works out in practice can vary depending on the composition of the attacking force, their artifacts and bonuses, their Clan Achievement bonuses, upgrade levels, etc.