You can project the edge of a stair to most AEC objects, such as walls or mass elements, but excluding multi-view blocks.
Projecting a stair edge to an object
Note: When projecting an edge of a stair that has an inner corner, it is clear how to project the outer edge of the stair to the selected object, but projection of the inner corner is ambiguous. Rather than attempting to guess the correct solution, these cases are not projected, and a dialog box is displayed with an explanation that edges of stairs with interior corners can be projected only to objects that extend beyond both ends of the stair.
The stair length is extended at its original width when the object (or polyline) does not extend far enough for the stair to project completely.