Standy Cabinet base Question

I’ve noticed that home kitchen and bath cabinets have an indented area that is approximately 3.5â€
or t n ttt for short (tnttt.com)
https://www.tnttt.com/
48Rob wrote:
The "false bottom" as you called it is a base for the cabinets that allows the toe kick and can be scribed and cut to fit uneven floors.
In some homes the dead space is used to run heating/cooling ducts and vents.
Rob