You will notice that when you are to select a variable from the dropdown there is an option for (DynamicVar). You will often want to use a variable to communicate some data between two tasks in a small scope and it would be tedious as well as unorganized to create a variable in the blackboard for such temporary use cases.
Using a Dynamic Variable allows you to directly input the name of a variable that will be created at runtime when it’s needed, without having one predefined in the blackboard. By convention Dynamic Variables should start with an underscore “_”. This is a feature that admittedly requires good naming organization on your part, but can be useful in some cases nonetheless 🙂