Bearing walls in the basement are walls that hold up the floor structure for the main floor of the house. They are built over the interior bearing footings described in the article Layout And Install Footings. They are built by cutting the wood for the walls to the exact length to match the plate that is bolted to the top of the foundation. The stair well needs walls all the way around it so the floor joists can be cut away to create the opening for the stairs.
The first step in framing the basement walls is laying out and bolting to the foundation the sill plate. 3" washers are used to prevent the house from coming off the foundation in case of an earth quake.
The top of the sill plate is the top of the double top plate on basement bearing walls. After the sill plate is on it is easier to determine the height of the basement walls. The length of the boards varies depending on how flat the concrete slab in the basement is.