#### 8.17.2.1 IfcShell

A type comprising different kinds of shell.

> NOTE Definition according to ISO/CD 10303-42:1992
> This type collects together, for reference when constructing more complex models, the subtypes which have the characteristics of a shell. A shell is a connected object of fixed dimensionality d = 0; 1; or 2, typically used to bound a region. The domain of a shell, if present, includes its bounds and 0 ≤ Ξ < ∞.
> - A shell of dimensionality 0 is represented by a graph consisting of a single vertex. The vertex shall not have any associated edges.
> - A shell of dimensionality 1 is represented by a connected graph of dimensionality 1.
> - A shell of dimensionality 2 is a topological entity constructed by joining faces along edges. Its domain, if present, is a connected, orientable 2-manifold with boundary, that is, a connected, oriented, finite, non-self-intersecting surface, which may be closed or open.

> NOTE Type adapted from shell defined in ISO 10303-42.

> HISTORY New type in IFC2x.

XSD Specification:

<xs:group name="IfcShell">
<xs:choice>
<xs:element ref="ifc:IfcClosedShell"/>
</xs:choice>
</xs:group>

EXPRESS Specification:

TYPE IfcShell = SELECT (
| IfcClosedShell);
END_TYPE;

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