How to understand the difference between GetRootcomponent and GetFirstSubFeature？

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GetRootComponent→ http://help.solidworks.com/2012/English/api/sldworksapi/Traverse_Assembly_at_Component_and_Feature_Level_Example_VB.htm

Traversing Assembly at Component and Feature Levels?

File = D:\Backup\我的文档\Draw\Assem1.SLDASM
    注解 [DetailCabinet]
    Comments [CommentsFolder]
    设计活页夹 [DocsFolder]
    光源与相机 [EnvFolder]
      环境光源 [AmbientLight]
      线光源1 [DirectionLight]
    前视 [RefPlane]
    上视 [RefPlane]
    右视 [RefPlane]
    原点 [OriginProfileFeature]
    Part1-1 [Reference]
    Part2-1 [Reference]
    配合 [MateGroup]
      距离1 [MateDistanceDim]
  +Part2-1 <默认>
    注解 [DetailCabinet]
    备注 [CommentsFolder]
    设计活页夹 [DocsFolder]
    曲面实体 [SurfaceBodyFolder]
    实体 [SolidBodyFolder]
    材质 <未指定> [MaterialFolder]
    光源与相机 [EnvFolder]
      环境光源 [AmbientLight]
      线光源 1 [DirectionLight]
      线光源 2 [DirectionLight]
    前视基准面 [RefPlane]
    上视基准面 [RefPlane]
    右视基准面 [RefPlane]
    原点 [OriginProfileFeature]
    Sketch1 [ProfileFeature]
    Extrude1 [Extrusion]
      Sketch1 [ProfileFeature]
  +Part1-1
    注解 [DetailCabinet]
      细节1 [DetailFolder]
    Comments [CommentsFolder]
    设计活页夹 [DocsFolder]
    Material [MaterialFolder]
    光源与相机 [EnvFolder]
      环境光源 [AmbientLight]
      线光源1 [DirectionLight]
    方程序 [EqnFolder]
    Surface Bodies [SurfaceBodyFolder]
    Solid Bodies [SolidBodyFolder]
    前视 [RefPlane]
    上视 [RefPlane]
    右视 [RefPlane]
    原点 [OriginProfileFeature]
    Sketch1 [ProfileFeature]
    Extrude1 [Extrusion]
      Sketch1 [ProfileFeature]
    Sketch2 [ProfileFeature]
    包覆1 [Emboss]
      Sketch2 [ProfileFeature]