One of the least documented features of the game is in the area of Sigils. There are a variety of effects, from bonus damage to on crit effects, but how each one interacts is still being researched. Lackofcheese over on reddit has a very nice write up of the Sigil system and explains some of the basics for how they work.
There has been a lot of confusion about sigils lately, and not without reason – the game really doesn’t give you much indication about how they work, especially when there’s more than one of them involved. The most important piece of advice to give here is that it’s easy enough to test all kinds of combinations for free by going to the Heart of the Mists and fighting Target Golems or Conquest NPCs.
Most of what I’m writing here I’ve gathered from my own testing in the Heart of the Mists, and it’s important to note that sigils may work differently in PvE/WvW than they do in sPvP, though I have seen no evidence to suggest that this is the case. Moreover, the situation may change with future updates to the game, especially since some of the following behaviours may not have been intended.
Check out the full write up here.