Member name	Value	Description
None	0	Default state of the scene, it imported successfully.
Incomplete	1	Specifies that the scene data structure that was imported is not complete. This flag bypasses some internal validations and allows the import of animation skeletons, material libaries, or camera animation paths using Assimp. Most applications won't support such data.
Validated	2	This flag is set by the ValidateDataStructure post process step if validation is successful. In a validated scene you can be sure that any cross references in the data structure (e.g. vertex indices) are valid.
ValidationWarning	4	This flag is set by the ValidateDataStructure post process step if validation is successful, but some issues have been found. This can for example mean that a texture that does not exist is referenced by a material or that the bone weights for a vertex do not sum to 1.0. In most cases you should still be able to use the import. This flag can be useful for applications which do not capture Assimp's log output.
NonVerboseFormat	8	This flag is set by the JoinIdenticalVertices post process step. It indicates that the vertices of the output mesh are not in the internal verbose format anymore. In the verbose format, all vertices are unique where no vertex is ever referenced by more than one face.
Terrain	16	Denotes the scene is pure height-map terrain data. Pure terrains usually consist of quads, sometimes triangles, in a regular grid. The x,y coordinates of all vertex positions refer to the x,y coordinates on the terrain height map, the z-axis stores the elevation at a specific point. TER (Terragen) and HMP (3D Game Studio) are height map formats.
AllowShared	32	Specifies that the scene data can be shared between structures. For example: one vertex in a few faces. This differs from NonVerboseFormat as that has internal meanings about postprocessing steps.

Reference

Internal.Assimp Namespace