The ObjectARX classes with an AcGi prefix relate to the graphics interface used for drawing AutoCAD entities. These classes are used by the AcDbEntity member functions worldDraw(), viewportDraw(), and saveAs(), which are all part of the standard entity protocol. The subWorldDraw() function, which is called by worldDraw() to provide the draw behavior specific to the custom entity, must be overridden by all custom entity classes. The AcGiWorldDraw object provides an API through which AcDbEntity::worldDraw() can produce its graphical representation in all viewports simultaneously. Similarly, the AcGiViewportDraw object provides an API through which the AcDbEntity::viewportDraw() function can produce different graphical representations for each viewport.
For more information on using AcGi classes, see Deriving from AcDbEntity and The Graphics Interface Library.