On the Vertical definition tab of the Wall: definition dialog, you can specify the location of a wall with respect to an active story.
The Wall upper level and Wall lower level lists include all the defined levels of an active story of a building. The vertical location of a wall in a building is defined by selecting a plane in which the top and bottom faces of the wall should be positioned (for example, Top - Story 2, Bottom - Story 2).
When defining the vertical location of a wall, you can define an offset from the lower or upper level of a story. If the offset equals 0, the definition line coincides with the upper or lower plane. Click to select a horizontal element edge that determines an offset value.
A sign convention for offset values has been adopted for both the lower and upper levels of a story: