Little raven head symbol next to abilities and items in the compendium?

[DUSTANDECHOES]

I know that when you unlock and item's set bonus a little symbol appears next to it to show that you've done so, but I have two question sets about this:

1. What does that mean for the hero abilities? What causes the symbol to appear?

2. How come some items have a sealed letter symbol next to them and some of them have a raven head symbol? Why is this not explained in the game?

[Fencer]

Sealed letter means you have seen the set effect for that item.
The other symbols correspond to difficulty selection, and you earn them by beating that difficulty while having that talent or having that item's set effect active.