Straight Back of the Wall
Some standards (e.g. AASHTO) require always the simplified straight back of the wall for pressure analysis in some specific cases.
The choice of the back of the wall in the "Stage settings" frame
The back of the wall is then considered as a line between the back points of the upper and bottom block. The earth pressure then acts on this surface under friction angle δ.
The simplified back of the wall is drawn in the "Verification" frame.
The earth pressure acting on the prefab wall - straight back of the wall
The earth pressure acting on the prefab wall - the standard approach
U.S. Department of Transportation, Publication No. FHWA NHI-07-071, Federal Highway Administration, June 2008.