When you are reading ecma262 spec it is easy because ecmascript specification only has one species of Realm and this is current Realm. Usually the current Realm tied with an active execution context.
When you're opening whatwg specification you sees that it has a few species of the Realms. It becomes more complicate now. You have to make a distinction among these species of the Realms for understanding of every of them and where and how they appears. This is complicate to do!
I'm trying to realize differences between the incumbent Realm and the current Realm. Examples would be best help.
Also there is the incumbent concept in whatwg spec (this concept directly tied with the incumbent Realm):
This corresponds to the most-recently-entered author function or script on the stack, or the author function or script that originally scheduled the currently-running callback.
To be honest it is unclear. I defenitely don't understand this definition.
Could you explain this concept on the examples?