A zero-thickness sheet is not a valid 3D geometric object in PDE Toolbox, but you could make a thick sheet instead and use one of its faces to define boundary conditions. Example:
>> gm1 = fegeometry(multisphere(10));
>> gm2 = fegeometry(multicuboid(5,5,1,Zoffset=-1)); % Zoffset shifts top face to XY plane
>> gm3 = addCell(gm1,gm2);
>> pdegplot(gm3,FaceAlpha=0.2);
>> gm4 = generateMesh(gm3);
>> figure
>> pdemesh(gm4);
As you want to apply boundary conditions along an internal sheet surface, you can use faceBC to set them on only one of the multicuboid faces. The multicuboid can be defined such that its top face (face F3 in the image) size, shape, and location coincide with those of the internal sheet surface you want to model. The rest of the multicuboid geometry will not affect the results as long as the same material properties are assigned to both cells. Also, as you use only one face for the boundary conditions, there is no need to make the multicuboid part thin. Giving it large enough thickness (1 in the example) relieves unnecessary constraints on the mesh generator.
