Spirals are curves that revolve around a point while increasing its length. In general, these curves are parameterized in the following way:
x = r(φ) cosφ
y = r(φ) sinφ

In the case of the clothoid and higher order transition spirals, the parametrization is:
x = ∫cos &Theta dl
y = ∫sin &Theta dl

where &Theta is the deflection or bearing angle and dl the differential of the spiral length.

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.

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.