This abstract entity definition is used for positioning and annotating elements that are used to position other elements relatively.

• EXAMPLE: A grid is a positioning element to position building components mainly in vertical structures, an alignment is a linear positioning element to position geographic and civil elements mainly in infrastructure works.
• EXAMPLE: An alignment is a linear positioning element for using a linear referencing method to position other items.

The local placement for IfcGrid is defined in its supertype IfcProduct. It is defined by the IfcLocalPlacement, which defines the local coordinate system that is referenced by all geometric representations.

• The PlacementRelTo relationship of IfcLocalPlacement points (if specified) to the local placement of the same IfcSpatialStructureElement, which is used in the ContainedInStructure inverse attribute, or to a spatial structure element at a higher level, referenced by that.
• If the relative placement is not used, the absolute placement is defined within the world coordinate system.

The 2D geometric representation of IfcGrid is defined using the 'GeometricCurveSet' geometry. The following attribute values should be inserted:

• IfcShapeRepresentation.RepresentationIdentifier = 'FootPrint'
• IfcShapeRepresentation.RepresentationType = 'GeometricCurveSet'

The following constraints apply to the 2D representation:

• The IfcGeometricCurveSet is an Item of the IfcShapeRepresentation. It should contain an IfcGeometricCurveSet containing subtypes of IfcCurve, each representing a grid axis. Applicable subtypes of IfcCurve are: IfcPolyline, IfcCircle, IfcTrimmedCurve (based on BaseCurve referencing IfcLine or IfcCircle) and IfcOffsetCurve2D.
• Each subtype of IfcCurve can have a curve style assigned with IfcStyledItem referencing IfcCurveStyle.
• Optionally the grid axis labels can be added as IfcTextLiteral, and they can have text styles assigned with IfcStyledItem referencing IfcTextStyle.

As shown on the image below, the IfcGrid defines a placement coordinate system using the ObjectPlacement. The XY plane of the coordinate system is used to place the 2D grid axes. 

As shown on the image below, the attributes UAxes and VAxes define lists of IfcGridAxis within the context of the grid. Each instance of IfcGridAxis refers to the same instance of IfcCurve (here the subtype IfcPolyline) that is contained within the IfcGeometricCurveSet that represents the IfcGrid. 

This definition cannot be instantiated.