Design layers might be the container for holding all of the building elements, but levels…
Vectorworks Tip #226 – Stories and Levels – Levels and Columns
Levels can be used to control the bottom bound and the top bound of columns. The levels can be in the current story, in the story above (the top of a column), or the story below (for the bottom of the column). This gives you a lot of flexibility to control the height of your columns automatically.