IfcProfileDef is the base class for all definitions of standard and custom profiles used within IFC. This class defines a standard set of commonly used section profiles by their parameters or by their explicit curve geometry.

• Parameterized profiles are 2D primitives, which are used within the industry to describe cross sections by a description of its parameters.
• Arbitrary profiles are cross sections defined by an outer boundary as bounded curve, which may also include holes, defined by inner boundaries.
• Derived profiles, based on a transformation of a parent profile, are also part of the profile definitions available.
• In addition composite profiles can be defined, which include two or more profile definitions to define the resulting profile.

Beams, columns, and similarly shaped building elements and their type objects may be associated with a section profile definition, combined with material definition, by means of IfcRelAssociatesMaterial together with IfcMaterialProfileSet and IfcMaterialProfileSetUsage. This way, building elements and element types with same section and material can share a common section profile definition and association. 

The profile definition in material association is required to be consistent with shape representations of the respective building elements. 

Profile definitions are used within the geometry and geometric model resource to create either swept surfaces, swept area solids, or sectioned spines. The purpose of the profile definition within the swept surfaces or swept area solids is to define a uniform cross section being swept:

• Along a line (extrusion) using IfcSurfaceOfLinearExtrusion or IfcExtrudedAreaSolid.
• Along a circular arc (revolution) using IfcSurfaceOfRevolution or IfcRevolvedAreaSolid.
• Along a directrix lying on a reference surface using IfcSurfaceCurveSweptAreaSolid.

The purpose of the profile definition within the sectioned spine is to define a varying cross sections at several positions along a spine curve. The subtype IfcDerivedProfileDef is particularly suited to provide the consecutive profiles to be based on transformations of the start profile and thus maintaining the identity of vertices and edges. 

An IfcProfileDef is treated as bounded area if it is used within swept area solids. In this case, the inside of the profile is part of the profile. The attribute ProfileType is set to AREA. An IfcProfileDef is treated as a curve if it is used within swept surfaces. In this case, the inside of the profile (if the curve is closed) is not part of the profile. The attribute ProfileType is set to CURVE. 

The ProfileType defines whether the inside (the bounded area) is part of the profile definition (Area) or not (Curve). 

If the profile is standardized by a norm or a catalogue, a reference to this norm or catalogue should be provided by means of HasExternalReference. This inverse relationship is used to associate an IfcExternalReference (notably IfcClassificationReference or IfcLibraryReference) with the profile. IfcClassificationReference is used to refer to a profile norm (a common standard or manufacturer's standard). In this case,

• IfcClassificationReference.ItemReference contains the formal profile designation from the norm. (On the other hand, IfcProfileDef.ProfileName contains a displayable name which may not necessarily be the same as the formal designation.)
• IfcClassificationReference.Name carries the short name of the profile norm.
• Optionally, the norm can be further described by IfcClassificationReference.ReferencedSource.

IfcLibraryReference is used to refer to a library which contains profile definitions. In this case,

• IfcLibraryReference.ItemReference contains the identifier of the profile within the library and is meant to be machine-readable (in contrast to IfcProfileDef.ProfileName which should be human-readable).
• IfcLibraryReference.Location and .Name or .ReferencedLibrary further describe the library.

If an external reference is provided, sending systems shall ensure that the shape of the profile definition object agrees with the definitions in the referenced classification or library. 

Usually, only subtypes of IfcProfileDef should be instantiated. In some special cases, e.g. if the profile object is used for purposes other than geometric models (e.g. for structural analysis models), it may be possible to directly instantiate IfcProfileDef and further specify the profile only by external reference or by profile properties. The latter are tracked by the inverse attribute HasProperties.