This .NET class wraps the AcBrFace ObjectARX class. 

The Face class is the interface class for Brep faces. This class defines the functions that are pertinent to the face level of a Brep, and thus serves as a mapping between AutoCAD face subentities and AcBr. It is also used to set the first face of a global AcBrBrepFaceTraverser or the face owner of an AcBrFaceLoopTraverser. 

Faces are primary brep elements and thus have associated geometry (that is, surfaces). This class provides an indirect link between Geometry classes, Solid3D, Region, and Body.

Autodesk.AutoCAD.BoundaryRepresentation.BrepEntity
    Autodesk.AutoCAD.BoundaryRepresentation.Face

Public sealed Class Face
Inherits BrepEntity

public sealed class Face : BrepEntity;

The intended use of the Face class is to create a Brep contexted face for dumping surface geometry and for providing interpretive context for parameter space curve and point geometry. 

The subentity path should contain the object ID of an AutoCAD solid, region, or body; its chained transforms; and a valid face subentity ID.