In Fluent, the threads listed in the Cell Zones and the Boundary Conditions have different use : - In the cell zone, you will find the threads of cells defined zone by zone. In each cell thread, you will be able to apply a source term, a value, or a new memory at the centre of the cell. - In the boundary conditions, you will find the threads of the faces that belongs to the different cell zones. If the face belongs to a named boundary condition, it will be included in a so called boundary condition. If the face does not belong to a named boundary condition, it will be included in a default interior (one default interior is created by Cell Zone). The default interior can only be defined as an interior type. That means that you cannot apply any boundary condition treatment on the faces that belong to this face thread. The utility of the default interior thread is mostly to define an ID on this faces thread to be used in a ANSYS UDF (User-Defined Function). Moreover, during the post processing in Fluent, computing a surface integral on this default-interior means computing a surface integral on all the faces of this thread. This makes usually no sense. What about 2D cases? In 2 dimensions in Fluent, the resolution of the equations are 2D but you always have to think in 3D for post processing purpose or working with thread in a UDF. That means that the 2D faces of a 2D mesh correspond to 3D cell zones corresponding to the 2D face extruded along a reference value or rotated in axi-cases. That means that you will only be allowed to compute a volume of these cells. That means that the boundary edges correspond to the face boundnary condition (also corresponding to an edge extruded or rotated). That also means that the default-interior will be composed of all the interior edges that are not defined in any named boundary conditions. For this default-interior, you will also be allowed to compute surface integral but you will have to be carrefull when post processing values in 2D on the default-interior face thread. For example, if you compute the surface of the default-interior face thread, you will make the summation of the area of all the faces defined: - planar cases : the length of each edge multiplied by the reference value - axi cases : the area of the surface created by rotation of each edge !! Computing the surface of the default-interior for 2D cases (using the surface integral panel) will not return the surface of the 2D mesh !!