Capacity called Block 1...4. 

It says "+x% defense when attacked". How different is it than just an effect with "+x% defense"? What's the meaning of the "when attacked"? It sounds redundant.

https://i.imgur.com/5ypTb5u.png 

There are ways to injure units without attacking them, such as walking over lava, weather, being burned, being poisoned, etc. Blocking is only effective against active attacks.

CaptainCobbs wrote:

There are ways to injure units without attacking them, such as walking over lava, weather, being burned, being poisoned, etc. Blocking is only effective against active attacks.

Does it mean that "regular" defense mitigates lava burning, weather and so on?

Maybe not all of those, but I know at least some of them are.