In general, each element that you can select individually in the Revit scene is translated to Unreal as a separate Static Mesh Asset. Some elements may be broken down further into smaller Static Meshes when they are made up of smaller parts, such as railings.
In all cases, the geometry of each Static Mesh Asset is set to match the dimensions of the Revit object at the time you export the file. Parametric settings and constraints are not carried over into the Unreal Engine. So, for example, if you move a floor up or down in the Unreal Editor, the height of the walls will not stretch or shrink to match the new location as it would in Revit.
If two objects belong to the same family, and if they have exactly the same parameter values, then both objects are represented in the Datasmith Scene as instances of the same Static Mesh Asset.
Datasmith relies on Revit's built-in tessellation services to create triangular meshes from your scene geometry, using the highest possible level of detail. In most cases, this produces adequate geometry for use in the Unreal Engine. However, the meshes produced by Revit can sometimes be overly dense, particularly on curved surfaces. For example:
Datasmith does not currently do any re-topologizing or simplification of these meshes on import. However, if you find these surfaces to be a problem in your Projects, you can try tools offered by the Unreal Editor for reducing the complexity of these meshes, such as the Proxy Geometry tool.