OMT Notation - Multiplicity - 0 or 1 (optional)
A Transmittal Root contains 0 or more Environment Roots.
An Environment Root optionally has a Feature Hierarchy.
An Environment Root optionally has a Geometry Hierarchy.
An Environment Root is aggregated by exactly 1 Transmittal Root.
A Feature Hierarchy is contained by 0 or 1 Environment Root.
A Geometry Hierarchy is contained by 0 or 1 Environment Root.
and is sometimes called “optional”