Blank and Unbound columnt difference

What is the difference between the blank and the unbound column?

I want to use it as additional filter and grouping value, that doesn't need to be written back in the Revit model. Important is, that the value doesn't dissapear when the schedule is reloaded and it keeps the relation to the actual element in case the schedule is sorted in a different manner.

Do we need to use Apply command in order to "confirm" the value? 

In terms of Revit logic, does it work as instance or type parameter?

Thank you for the explanation and any tips.