Can some1 explain Flat magic pen & % magic pen.

Flat penetration (be it magic or armor) is great against low armor/MR targets. Some completed items that are good against low armor targets: . Some items that are good against low MR targets:

Percent penetration is great against those who are stacking LOTS of armor/MR. This is where the items / for armor and for MR shine.

I hope this helps.

Flat magic penetration allows you to ignore set amount of your enemies magic resistance, i.e. Sorc Shoes will always penetrate 15 points.

Percent magic penetration allows you to ignore certain percent of your enemies magic resistance, i.e. Void Staff will penetrate 35 MR against 100 MR target, an 70 against 200 MR target.

Flat penetration is better against enemies with low magic resistance. Percentage magic penetration is better against high magic resistance targets, but it stacks well with flat penetration, so you can as well get both if you need them.

Flat pen just removes the number and thats it, Assassins like Flat pen since it makes squishy champs even squishier to them.

% Pen is a counter to tanky champs since it removes a chunk of their defenses, 20 armor pen won't help against a tanky champ with 200 armor, but an item with 20% armor pen is amazing since that is now 40 removed, which will actually make a difference since that is nearly an entire tanky item removed. That is why is so damn powerful, it basically cuts armor in half, on top of adds a nice damage boost against tanky champs. That will take away about 90 armor from a 200 armor tank as well as give them 15% more damage which for an ADC, you are talking about 100 more a hit which isn't something to scoff at.