An IfcEllipse is a curve consisting of a set of points whose distances to two fixed points add to the same constant.
The inherited SELF\IfcConic.Position.Location is the center of the IfcEllipse, and the inherited S_ELF\IfcConic.Position.Position.P[1] is the direction of the _SemiAxis1.
Definition of the IfcEllipse within the a threedimensional position coordinate system is shown in Figure 1. It is placed within the object coordinate system of an element of which it is a representation. 

Figure 1 — Ellipse placement and parameterization 
NOTE An elliptical arc segment is defined by using the IfcTrimmedCurve with BasisCurve
NOTE Definition according to ISO/CD 1030342:1992
An ellipse is a conic section defined by the lengths of the semimajor and semiminor diameters and the position (center or mid point of the line joining the foci) and orientation of the curve. Interpretation of the data shall be as follows:
C = position.location x = position.p[1] y = position.p[2] z = position.p[3] R1 = semi axis 1 R2 = semi axis 2
and the ellipse is parameterized as:The parameterization range is 0 ≤ u <≤ 2π (0 ≤ u ≤ 360 degree). In the placement coordinate system defined above, the ellipse is the equation C = 0, where
The positive sense of the ellipse at any point is in the tangent direction, T, to the curve at the point, where
NOTE Entity adapted from ellipse defined in ISO 1030342
HISTORY New entity in IFC1.0
#  Attribute  Type  Description 

IfcRepresentationItem  
LayerAssignment  SET [0:1] OF IfcPresentationLayerAssignment FOR AssignedItems  Assignment of the representation item to a single or multiple layer(s). The LayerAssignments can override a LayerAssignments of the IfcRepresentation it is used within the list of Items. > IFC2x3 CHANGE The inverse attribute LayerAssignments has been added. > IFC4 CHANGE The inverse attribute LayerAssignment has been restricted to max 1. Upward compatibility for file based exchange is guaranteed.  
StyledByItem  SET [0:1] OF IfcStyledItem FOR Item  Reference to the IfcStyledItem that provides presentation information to the representation, e.g. a curve style, including colour and thickness to a geometric curve. > IFC2x3 CHANGE The inverse attribute StyledByItem has been added.  
IfcGeometricRepresentationItem  
IfcCurve  
Dim  INVALID  The space dimensionality of this abstract class, defined differently for all subtypes, i.e. for IfcLine, IfcConic and IfcBoundedCurve.  
IfcConic  
1  Position  IfcAxis2Placement  The location and orientation of the conic. Further details of the interpretation of this attribute are given for the individual subtypes." 
IfcEllipse  
2  SemiAxis1  IfcPositiveLengthMeasure  The first radius of the ellipse which shall be positive. Placement.Axes[1] gives the direction of the SemiAxis1. 
3  SemiAxis2  IfcPositiveLengthMeasure  The second radius of the ellipse which shall be positive. 
ENTITY IfcEllipse SUBTYPE OF (IfcConic); SemiAxis1 : IfcPositiveLengthMeasure; SemiAxis2 : IfcPositiveLengthMeasure; END_ENTITY;