Maybe this term is so usual for gardeners.
They usually grow short plants under the higher crops.
Look, I also practice this method on my terrace garden.
On a container, I have three varieties.
The main crop is tomato, and the additional are lettuce also a pot of garlic chives.
How about this one....
I will pull out the red amaranths soon, and cook it.
They have different period on harvesting stage.
I planted the main crops on the center of containers.
The main crops means plant that have longer period on production life time, than the 'additional' crops.
The additional crops are harvested earlier before the main crops.
When the main crops need more space, the life time of additional crops is over.
So, we use the space effectively.