As per one case study, Etabs considers Slabs maintaining slope more than 3.63/10 as Wall area object.
Whatever it considers that is not our concern, concern is-we have to be careful while using stiffness modifier for slab & wall (as they are different for each, for stair we will use modifier same as Slab). Although Etabs may consider sloped slab as wall area object for meshing but it doesn’t have any different effect on analysis. So, no need to worry about what it is considered by Etabs (as floor area or wall area for meshing). Some newbies try to use one way slab(in this case the slab has to be defined as membrane) for stair, but it’s wrong. We model stair to provide realistic scenario in the analysis where stair provides additional rigidity or stiffness to the structure with its bracing-like action. If we model the stair as membrane(Only in plane stiffness and no out of plane stiffness) then it’s useless. So, better practice is-“we can model stair slab as shell element to provide possible rigidity in the structural analysis and we can design stair manually”. Some other experts suggests not to model stair for avoiding confusion or complexity. So, it’s depending on you in which approach you will go.
So the summary is, if we model stair as shell in etabs (whether it considers floor area or wall area) then we don’t need to worry about it.
To read more about shell & membrane you may check this blog- https://decode-bd.com/what-is-the-difference-between-membrane-and-shell/