A user surface isn't part of the mesh, it's added into Fluent and data is interpolated onto it from the mesh. An interior/wall/inlet etc is part of the mesh as it makes up part of the geometry.Â
User surfaces aren't transferred, so you need to create it where ever you want it. We can use the surface integrals in Fluent to get the mass flow, but if it's a value to want to know before you start the modelling I'd break the domain up to include that surface in the original model. So your domain might contain multiple faces in geometry that are connected to ensure a conformal mesh.Â
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