That is where you dig and pour a foundation, lay up 4-5 courses of block, fill in the box with dirt, pour a slab and then build the house off of that. It gets your finished floor above the FEMA flood plane without having your foundation on fill. Then they backfill around the stem wall to make it look like you are built at grade.
My wife was the queen of the stem walls, building more stem wall houses than anyone else at Centex in SW Florida. (over 100). It was just because of the swamp they assigned her as her patch. They brought something like 1000 acres up 4-5 feet with fill.
