State Machine Graphs can not reference scene objects due to living in the Project Panel. To overcome this, we've created the Blackboard, where the developer writes down the input fields he/she'll need and these are filled at runtime.
As you may have noticed in the image above, to access the value of a Blackboard entry is done as if it was a Local Variable. This is because under the hood, the State Machine component generates a new Local Variable component and fills it with all the entries from the Blackboard.
Once the Blackboard is filled with some entries, the State Machine component is automatically updated, adding new entries and removing those that no longer exist in the Blackboard.
For example, let's say our game has a Guard that can attack anything when it has a target defined. We could create a target entry in the State Machine and ask if it's different than null. If so, follow it and attack when it's within in range.
(Can't have a Local Variable with the same name as a Blackboard entry)
Because the StateMachine component generates a Local Variable object with the Blackboard entries, it will throw a conflict error.
Invoker is a variable automatically generated and it will always have the game object where the State Machine is as reference.